Personally I am a Glock fan, so the G21 gets my vote.
Realistically all three you listed will serve you a lifetime, almost regardless of how much abuse they get. As others have said, how it fits your hand, and/or how well you shoot it is the biggest factor that should affect your choice. With limited stock available in your area, maybe post a "wanted to shoot" thread, and see if someone on here is local enough to help out with a test fit and or shoot at a local range.
If that is not an option, my next best idea is to buy all 3 on the EE and do the comparison yourself. Once you decide, sell two off and keep the third. Even with a small loss on each and added shipping, you will have spent less than the cost of buying it new, and you know you got the best gun for you.
Even if you sell all three after your comparison, because you want to own a brand new gun, you will only be out $150-200 overall. Whereas if you buy a new G21 for example $850+shipping+tax then don't like it then sell it on EE for $700-$725 you will be down $200 and still left to decide between the M&P and Sig, needing to buy & try one or both of them.
I've bought lots of used guns on here with no issues of note, in fact I am waiting on the CFO to approve my used G21 G4 I recently bought on EE. Of the other 4 Glocks I own, two were bought used, and the other 6 pistols I own, 3 were bought used.