I really want it
 

新しい車が欲しくてしかたがない　・　君に新しい車を 買ってほしくてしかたがない
新しい車が欲しくてしょうがない　・　君に新しい車を 買ってほしくてしょうがない
新しい車が欲しくてたまらない　・　君に新しい車を買 ってほしくてたまらない
I was told they all mean the same. Is there any difference in usage or I can pick the one I want?

Those are essentially the same if not "completely" and I am being anal here.

たまらない is more emotive and it expresses the speaker's desire more strongly than the other two. However, this is not something anyone would realistically expect a non-native speaker to be aware of.

Thanx...since that source I got it from more than once lead me astray hee hee.

めっちゃ車ほしーねん　　おんどれ車こーてくれや
ごっつぅ車ほしーんねん。　車こーたってやぁ
車ほしかぁ～　　こーてくりゃい
ブーブーほちーのぉ　　かっちくりぃ