The only difference between 3rd and pickup is angle:
they're actually the same mesh, only imported using different parameters. The 3rd person is usually slightly angled upward, and also of course rotated 90 in 'roll' so it's not held as if it were lying flat on the ground.
It depends on what the weapon is. Check out the import classes for the original weapons for guidance.
And DaveW: we often DO use the same base mesh for first and third/pickup for the HDTP weapons, as in "make a weapon (3rd/pickup), then just add some hands and delete backfacing polys (first)" so what you see on the floor is the same as what you see in your hand.