Is there a musical key with the A maj, F maj and G maj chords, please?

Why not post a link of the music your working on. At least we will get a better idea of which key is better for you to use.

I think the key of G would work, but it might be a little odd.

Not enough info.  What else is going on in this song of yours?

No western key. (that doesn't mean it is wrong)

These chords require the consecutive notes B, C, C# and D. No key has a run of more than 2 notes let alone 4 notes.

if you were notating it, you'd need to use the fewest modifiers if you used G, which is missing only the C#.

It would help to know what other chords are used with these ones.

It is possible that the song is in C major. A major does not normally appear in songs in the key of C as the  A minor chord would be far more likely, but there are no rules against including chords from another key. They are used to create a particular sound, to tweak the expected pattern.