Thank you for the advice - I really appreciate it
All three bikes you've listed look ideal for your son - why not let him choose? When my kids (now 17 and 21) were younger I always narrowed it down to a choice of bikes I felt were suitable/of good enough quality and that I was willing to pay for and then let them make the final choice so they got a bike they were happy with (having said that, most of their bikes were SH). Agree about the sizings you've listed. For your middle child I'd go with a bike with 24" wheels if you can (check the standover height isn't too high though). It'll last longer and can then be passed on to your son when he outgrows the 20". Don't know if it's still the case but 24" bikes used to come in a choice of frame size - we went with the smaller framed 24" bike rather than a 20" and it lasted well. She did look tiny on it though! Friends with younger kids than mine highly recommend Frog and Islabikes as being well designed for younger kids (and retaining their value when you come to sell them on). Sometimes kids can struggle to operate the gears and brakes - I had to get the brake levers changed on my son's 20" Edinburgh Bicycle Coop bike because his hands were too small to reach. EBC switched them for levers with adjustable reach for no charge - I can certainly recommend their customer service!
Yikes - what area is that?
what ever one looks the best so it can be stolen at knife point by some low life from your child the minute they go out with friends, that seems to be the trend in my area anyways