Alma - the Hungarian word for 'apple' is 'alma'... but I have come across it used albeit as a pretty rare name. It's pretty and understated.
Alva - makes me think of 'ulva'... like... sea plants. I'd call this my least favourite.
Alba - is okay, but there's something a little off-putting about it.
In order of favourites it would be
Alma,
Alba,
Alva. I can't say I'm really sold on any of them, though.