Thanks for your questions. This topic has actually sparked a lot of interest and I am hoping that perhaps I can get a canonist or two (since I am not one) to weigh in here on the idea of lauras of canon 603 hermits especially as regards the origin of the canon and commentaries written on it. Your own first question is about the benefits of not having more than three hermits. When I wrote about that number I was thinking of a limit placed on groups of hermits by the Bishops in Spain (if my memory is correct in this). At that point I wrote about not allowing a colony of canon 603 hermits to morph into a community because that is a different vocation and contrary to the notion built into canon 603 by its authors.
But you can think about the problems that can occur with groups that get larger than three. With three people arrangements for chores or charges, hours of activity and silence, finances, the way visitors are handled, assuring silence and solitude for others and maintaining a prayerful atmosphere, times for communal liturgy (if there are any), shared lectio divina (if chosen), and meals, all of this and more can be handled with a simple meeting of the hermits. It is an optimal number for dealing with differences and coming together in a way which does not infringe on authentic freedom. Not so with larger groups.
Remember that all canon 603 hermits write her/his own Rule and this is approved with a bishop's decree of approval on the day of profession. While I don't recommend a hermit itemizing every do and don't when she writes a Rule, I do recommend a hermit writes her Rule in light of the vision she has of canon 603 life in the Church. Each hermit knows how God works in her life and what she needs for this; she will know the central elements of canon 603 and what they require of her in her daily living, and she will know all of this on the basis of lived experience before she writes her Rule and is professed. In each Rule written by each hermit there will be a wisdom the others won't necessarily accent. All of this experience and wisdom glorifies God and a laura has to be flexible enough to accommodate differences in emphasis and praxis which stem from the unique ways God works with each soul. This just naturally becomes harder as a group gets larger and the differences in one embodiment of c 603 begin to look like departures from what some others call "eremitical life."For instance, I define stricter separation from the world in a particular (and theologically sound) way which embraces silence and solitude and continual personal formation in Christ (growth in holiness towards what canon 603 calls "the silence of solitude"), but also allows me to use a computer, access the internet, and be active in a parish community (though in a limited way). Others without my specific experience and sensibilities (not to mention a wise and experienced director) could be unduly tempted by these things, fail to use them prudently, or, because they don't have the experience of handling these wisely, actually harm their vocations (and those of others in a laura) with them. As the laura grows in numbers it becomes harder to allow the very freedom eremitical life is meant to nurture and protect --- the very freedom which is a hallmark of both eremitical life and the Holy Spirit. (Please note, again, freedom is not the power to do anything we want whenever we want; it is the power to be the persons God calls us to be. To force solitary hermits to submit to a common Rule instead of, or as a replacement for a personally discerned and authored Rule is contrary to true freedom in regard to c 603 vocations.)
In such circumstances, prohibitions applicable to everyone become necessary (these are no longer the mutually agreed upon house rules which serve both freedom and charity) and in such cases people will also begin to look for a single person to act as the authority and "lay down the law." (While all the hermits I know personally would certainly be able to take on a leadership role if necessary, none of them/us feels we are actually called to this. It would require significant discernment within a proven environment to make such a choice.) The group will also require larger facilities and grounds --- which most hermits cannot afford and most dioceses, quite rightly, are neither able nor willing to provide for c 603 hermits.This is merely the tip of a very large and difficult iceberg; it is simply much easier to accommodate one or two other hermits and the vision they each have of canon 603 life. As alluded to above, one signal piece of wisdom of canon 603 is its legislating that a hermit write her own Rule rooted in her own long-experience of God at work in her life. This individual Rule and the ability to write one is key not only to the vocation itself, but also to dioceses discerning the reality of the vocation and assessing the formation of canon 603 hermits. In a laura, solitary hermits' individual Rules not only do not need to be superseded by a single Rule and superior, they should not be superseded in this way since it can destroy the very calling they have lived for years in discernment and personal formation to embrace and embody. Again, it is a different eremitical vocation.
I'm not taking your questions in order, and in some ways my responses make a direct response impossible. The area of formation is one of these so let me turn to that. As envisioned in light of canon 603 --- which regards the solitary eremitical vocation as preeminent and is entirely geared to it -- a laura of c 603 hermits would not be responsible for the formation of other hermits. A laura is composed of already-professed hermits who have been formed on their own (and often in other forms of consecrated life first) and who have demonstrated the capacity to live as solitary hermits in the same way. It seems to me that a c 603 laura would not accept candidates -- though members with genuine expertise might certainly be able to serve as resources, spiritual directors, or delegates for hermits approaching (and subsequent to) profession. Candidates (the term is used in an informal sense only since c 603 does not provide for "candidature" per se) might occasionally make a desert day with the c 603 hermits, join them for communal prayer once in a while, or meet for regular direction, but they would live elsewhere and would be responsible for securing their own formation, both initial and ongoing. This is important because it is an ongoing need for the whole of the hermit's life, and because it is intrinsic to canon 603 life in particular.It should but, (because of the way the notion of a laura has been misused from time to time) cannot go without saying that should a diocese have more than three c 603 hermits or those who would like to pursue such a vocation, there must be absolutely no requirement whatsoever that such persons join an already-existing laura or constitute themselves in such a way in order to be professed. Such a requirement is entirely foreign to the spirit and letter of canon 603. A laura is established for the benefit of the solitary eremitical life, not to co-opt it or transform it into something else. And yes, as you say, three diocesan hermits might well be all a diocese ever sees given the relative rarity of the vocation. (In fact, given the requirements I have stated here regarding freedom and formation, for instance, it is far more than most dioceses will ever see.)