Great questions.
It is worth knowing that definitions and markers for Third Path vary, and we are not all quite on the same page in these regards, as well as having different emphases as to what is important for Third Path. Thus, you may have to get used to a little friendly controversy regarding what is important and essential.
I tend to emphases the following things for Third Path:
1) Have completed at least 3 full, new insight/path cycles. This is not necessarily a guarantee of third path, as counting insight cycles is problematic and sometimes inaccurate, as one can review old territory after having crossed the A&P of the next path, get a Fruition (from the path you have already attained), think that Fruition was of the new path, and believe you have finished the next path, when really it was just review from the previous one. This is more common than most think. Now it is true that people who come up in traditions that don't emphasize path cycles or maps can be anagamis but just don't know what to count, and so for them this criteria is not as useful.
2) Have a substantially increased appreciation of this being ultimate reality, the truth of the dharma being here and now, of things being empty/luminous/causal in real-time when walking around. This understanding can deepen as third path deepens, but one should have a substantial appreciation of it in ways that people in second path simply don't. Kenneth doesn't emphasize this as much as I do. More panoramic and spacious perspectives should be more readily available to the mind in times of difficulties and in general.
3) There are the jhanic criteria, of which I prefer Nirodha Samapatti as more hard evidence than the Pure Land Jhanas. Not all anagamis will be able to get Nirodha Samapatti, so this is not as good a criteria, but if you can get it, you are very likely third path.
How important is it to have hard jhana skills in order to get Third Path? Well, this is a complex topic. The texts and commentaries do make some mention of the relationship between anagamihood and concentration mastery, but there are also "dry insight workers" who don't have much in the way of jhanic skills.
I personally had been into the formless realms solidly right after stream entry when playing around, but I was on retreat and my practice was very strong on that retreat and particularly right after getting stream entry. However, when I went to back to working and daily life, my concentration skills were moderately diminished, such that the 4th jhana in some moderate strength was pretty easy to get, but formless realms and more solid concentration attainments were much more elusive and spotty. However, once I got Third Path, which I did in daily life on a few hours a day of practice having crossed the A&P of that path on a short retreat, I suddenly had much easier access to the formless realms in daily life and was soon able to get Nirodha Samapatti also in daily life. Thus, I didn't have really hard jhana before getting Third Path.
In short, while there is something about the width and openness of the formless realms, particularly the first two, that does something good to the brain and helps with Third Path, it is certainly not necessary to have them mastered before going for Third Path, though, if kept in perspective, couldn't hurt and might help, obviously.
Is that helpful?
Daniel
