I think 3 planets without the Sun and Moon is fine for a stellium, but generally 4 planets is what is considered an "authentic" stellium from what I've read. I personally would disregard the stellium label altogether and just look for what houses seem to garner the most activity because that would showcase your prominent areas. For instance, I have a splash chart so I'm going to feel a lot of connection to many houses in general, but finding emphasis on my dominant houses is harder to gauge. However, after thorough examination over the years, I've come to the conclusion that my 4th and 10th houses are the most important in my chart due to my top dominant signs residing there as well as stronger aspect placements regarding the angles taking place there too. Most likely, if you have a lot of planets in one house over the other, it usually means there's more of a fixation there over other houses, so it would be an important factor to at least consider in your chart.
To answer your second concern, some astrologers believe that early degrees is an embodiment of the sign in its purest form, but simultaneously indicates inexperience and new beginnings. The later the degrees, the more mature the sign. I have read that a person with many later degree placements is indicative of someone being an "old soul" versus early degree placements as "new souls." Think of a newborn fawn just learning to walk on its gangling legs versus an older, sturdier deer. The 29th degree is an anaretic degree which means it's at its peak point and can take on traits of the next sign, but anything after 0 has no cusps and is solely in its own sign. However, 0 degrees is still considered a critical degree and therefore your Mercury would be very potent, but there will be a lot of bumps and grinds on the way. Not sure how significant this all is, but it's another interesting theory to look into, I suppose. Hope this helps somewhat.
Btw, fellow Cancer Sun, Pisces Moon here.
