Yes, the lowers are interchangeable.
I would GUESS that some would want a new 3+ lower in order to have an "AR 15 like" manual of arms which I can understand. The bolt release is also a tad easier to deal with and you only have to nudge the tab forward with the trigger finger to activate.
But... the best mags are Magpul, and they will NOT drop free from the 3+ - why I went the other way and ditched my 3+ lower and put on a 3.
The other thing is that if you index your firing finger hard enough when not firing, you can accidently release the mag.
I guess it depends on personal preference. Having one of each, 3+ and Evo 3, I prefer the AR-type lower receiver and the 3+ CZ factory mags.
Problem is, it’s usually very hard to find the lowers and internal parts of either in stock anywhere. From what I’m seeing, to buy a new lower and internal parts for either would cost @ $300 to $500 — if you could even find all that in stock, which is not likely.
So… if anyone out there wants to swap their assembled 3+ lower for an Evo 3 assembled lower and CZ 3+ 30-rd mags for Magpul 35 round mags (many brand new in the plastic wrap), I’m game!!!