ともだち　が/を/に　まてますか？

Hi,

Which sentence is correct for "Are you able to wait for your friend for one hour?"

1. ともだち が いちじかん まてますか。
2. ともだち を いちじかん まてますか。
3. ともだち に いちじかん まてますか。

Thank you very much

2. (Although I would phrase it 'Can you wait an hour for your friend?' in English.)
1 asks 'Can your friend wait for one hour?' and 3 is odd.

#1 is also correct, since まてる is the potential form of まつ and が indicates the object of some kind of verbs or adjectives(potential forms, -tai forms, state verbs or adjectives which express emotion/sense).
Of course ともだち can be the subject of まてますか, as already explained, depending on the context. (Therefore I, too, recommend to use #2 to avoid ambiguity.)