Well I've done a bit of research (yes I have way too much time on my hands) so here we are, this should be a comprehensive list of all the third drivers and how many times they performed the duty since 2003.
2003:
Jaguar: [No third driver]
Jordan: Baumgartner (2), Motoyama (1), Wirdheim (1)
Minardi: Bobbi (1), Bruni (5)
Renault: McNish (15), Montagny (1)
2004:
BAR: Davidson (18)
Jaguar: Wirdheim (18)
Jordan: Doornbos (3), Glock (14)*
Minardi: Leinders (17)
Sauber: [No third driver]
Toyota: Briscoe (6), Zonta (12)
2005:
Jordan: Doornbos (9), Kiesa (7), Montagny (1), Yamamoto (1)
McLaren: de la Rosa (12), Wurz (7)
Minardi: Nissany (1), Toccacelo (3)
RBR: Klien (4), Liuzzi (13), Speed (2)
Sauber: [No third driver]
Toyota: Panis (1), Zonta (18)
2006:
Honda: Davidson (15)
MF1: Mondini (9), Sutil (2), Winkelhock (4)
RBR: Doornbos (15)
Sauber-BMW: Kubica (12), Vettel (2)
Super Aguri: Montagny (2), Yamamoto (3)
STR: Jani (15)
Williams: Wurz (15)