XCode 3 is quite old, it can run on 10.4 (Tiger, pre-Intel) I believe. Snow Leopard can run up to XCode 4.3 (or there about), and Lion can go up to 4.6.
From what little I know about Phonegap, it's not as good as it sounds (IMHO), there's a price to pay for writing in Javascript and bypassing all the rest.
My 2 cents, research this topic well before diving in.