They may be different but all of them are forwards that play both on the wing and centrally. They'd be competing for the same positions if they were in a squad together. I think Arnautovich and Shaqiri are both better options than Sigurdsson, and that it would be great to have both Arnautovich and Shaqiri in the same side.
Arnautovic is very much a classic inside forward.
Shaqiri is more of a winger, who can play central.
Sigurdsson is a no.10.
I'd rather have Gylfi for the no.10 role than any of them.
If we were wanting an inside forward, Arnautovic.
If we were wanting a winger, Shaqiri.