The reason that there are three is basically because Steve managed to get the UMCA to agree rules for what would need to be done for validation (Guinness refuse to take any more entries as they think it's a 'record too dangerous to break'). It's a bit like the fact that there are so many hour record attempts now because of the rule changes at the UCI last year. Now there is a record and a body that will validate it, other ultra-distance cyclists have decided to have a go.
I don't like the idea that other riders are starting later, but it does mean that Steve has one record to break, Tommy Godwin's (which the target he has had since childhood), whereas the other two have Steve to beat as well (if he manages to take the record). If Steve passes TG's record then he will always be a distance record holder, whatever happens afterwards with others.
From what I understand, Kurt Searvogal is going about the ride a different way, in that he intends to ride faster, shorter days. He's ridden RAAM, so is no stranger to ultra distance events. William Pruett is intending to do this around his job as a cycle trainer, though his attempt so far doesn't seem to make any sense as he's hardly riding anywhere (where he's based it's about the same temperatures as here at the moment)
With respect to location, Steve doesn't have to stay in the UK if he doesn't want to (in fact he's planning to ride Paris-Brest-Paris this summer if he can get it to fit). Changing location does lose you time in transfers though, and is logistically more difficult than staying close to home.