Keep in mind that the OEM 3000 Sector is only 70° and just barely at that. It’s a really good sector otherwise though we have several deployed and more to to go. We are also big fans of the RF-Elements asymmetrical horns though too and right now if you need 90° with a 3000 two RF-Elements 45° Asyms would do it (you have to point them so that they do not overlap, so two 45s would give you 90, two 90s would give 180°). See this post ePMP 3000 with RF Elements Horn - #10 by Dmitry_Moiseev
While 3k may be a short distance for most radios that may not be the case for the little 14 and 13db radios. It will also depend a lot on what frequencies you use and what the power limits are in your regulatory domain. I have no personal experience with the 300-13’s other than a few we used for very very short (shooting across a 200ft parking lot kind of thing) PTP links. No idea how well they would work at 3km PMP, we don’t use anything below F300-16 for customers and those are all around maybe 2km but usually much less. The 300-19’s we run up to maybe 4km or so and pretty much F300-25 for everything up to 10km.
While you can mix and match N and AC on ePMP I wouldn’t if I could avoid it. While technically there shouldn’t be any problems they have repeatedly introduced, fixed, re-introduced, fixed, changed, fixed, re-introduced a whole host of firmware bugs that cause problems specific to AP’s with mixed clients on it. Also as pointed out, N can’t do MU