Re: Starting with all 3 starters?
I usually use all three starters at some point, because they tend to be my favourite Pokemon of any particular generation. I think they'd definitely be useful and you should be able to get through the game pretty easily with all three. As for IVS and natures etc., they're not really important if you're just playing through the game. If you like all the starters then you should go for it, but if not, I'm sure you could find and train other Pokemon who would end up just as good if not better. =)