I can only think of two, and both their named begin with Q.
tenuous - but the holographic doctor was in a Next Generation film, appeared in Doctor BAshir, I presume as well as being in Voyager.
Of course these were different installations of the EMH.
Didn’t Deanna appear in the first episode of DS9? Been a while…might not have been.
If you count the Doctor, you might as well include Majel Barett as ‘computer voice’ too.
Definitely not - Picard was the only TNG guest star on the premier of DS9. (O’Brien of course was cast as a DS9 regular.)
Indeed, Quark is one of the characters I mentioned whose name begins with Q. 
The Doctor doesn’t count, since he was only on the Enterprise E in the movies, but I’ll accept the Computer Voice.
I’ve read the synopsis of ‘Firstborn’ but it doesn’t mention Quark. What part did he play in this episode?
Riker calls Quark to call in a favor for something – I don’t remember what.
Riker was in all three. Well, sort of. His transporter-accident-clone brother Tom showed up as a Maqui in DS9, then got called in by Q as a character witness in Voyager.
So, if you get to claim different installs of the Doctor, Riker counts, too.
Q was in all 3