Carson, Colin, and Christian all sound very similar with the same beginning and ending. Of your current choices, I'd vote Preston by default just to vary it a bit, but even that is a little too rhymey for my taste. I'd avoid all names starting with C or ending with n completely.
Re: Name for third boy
if that doesn't bother you, go with Christian.
Baby #2 Due 3/7/20
Conrad
Jasper
TTC since September 2012