Merckx I know for sure. I'd be pretty sure about Mig. He rode the Vuelta several times and in or near his prime it woul dbe a huge long shot for him not to have won a TT in any single Vuelta entry. With Hinault winning the Vuelta twice he also is a good bet. Anguetil similar to Mig. He had to have picked up a Vuelta TT win if nothing else.
Edit: Add Figneon. '87 Vuelta stage 19. Plenty of Giro and TDF stages.
Look up this site:
memoire-du-cyclisme
Find it in google and use the translate option. For any retired rider who whould have a stage win in all 3 you will be able to find their results Look under 'Awards' (the only word where translating from Frence to English turns out to make it harder for an English speaking cycling fan).
There should be many riders who have a stage in each post WW II.
I think in recent times some sprinters have won a stage in each during the same year. Try Chippo.
EDIT 2:
Go to this site:
http://www.cyclinghalloffame.com
Navigate thriugh the Grand Tours to the Vuelta Records and try all riders who have most wins , sprinters or KOM jersies. Good chance any of those won a stage in each of the Major tours.
Also I'd say Rudi Altig likely has a stage in each, since in 62 he won two jersies in the Vuelta and I happen to know he won the TDF prologue in 1969.
I think there are more than 30 riders.