In the UK:
O/U are far ahead in both use and sales
S/S second, in use, though in new gun sales I would think auto possibly higher
Auto - third in use, but possibly second in new sales?
Pumps are seldom seen in the UK

On the clays ground you will see 90% O/U, about 5% each s/s and auto
On the shooting field it can vary a lot between O/U and s/s, but mainly O/U for younger generation, s/s for more traditionalists

Autos used more in wildfowling (lead free shot issues) and pigeon shooting

John