There's no reason why this lens shouldn't work on your FM3a, as Ian C has already mentioned, it's only a selected few Nikon lenses which have an extended rear element which won't mount onto the body without locking up the mirror.
However, the Ken Rockwell website mentions problems viewing the meter when using this lens on the old FE and FE2, so I'm not honestly sure if the same would apply with the FM3a as I've never used this lens personally. Otherwise, mechanically the lens will work fine.
As for your other wide-angle lens choices, as you're already using the 50mm you may like to consider the 28mm as your next step and it is a nice sharp lens. You won't go wrong with any of them actually, it just depends on the type of photography you like to do.