you can put the sim in any unlocked phone too!!!its only £10 to get them unlocked!!
No you can't.
Three sims will only work in 3G phones, 2G ones won't see them (for the most part, as in all things there are a few variants from software updates)
Also some of the Three sims work on a different voltage to the norm, so won't work in some 3G phones.
If you can try it first, do.
If you can't, stick with a Three phone.
Oh yes, some phone unlocking is FREE
Some costs less than a tenner, some is a lot more