A few things:
1) If every mod had to own all 3 systems, then there'd be very few mods.
2) Some people can own a system and dislike it: Lack of specific games on Wii/PS3, lack of hardware quality on X360. Major issues about specific features. I can say I really disliked my N64. I bought it for RPGs, and played all of 2 RPGs on the thing.. :-\
As for what mods own:
Mrstickball: X360
TalonMan: Wii
TheSource: Wii
Sqrl: Wii
FishyJoe: Wii
DKII: Wii
BenKenobi: Wii
Rocketpig: Wii/PS3/X360
TWestern: Wii/PS3/X360
SSJ12: PS3
BengaBenga: Wii
shams: Wii
MontanaHatchet: Wii/X360
Naznatips: Wii
Bodhesatva: Wii (I think?)
Stof: Wii
ioi: Wii/PS3/X360
Totals:
15 Wii (11 exclusively)
5 X360 (1 exclusively)
4 PS3 (1 exclusively)
Back from the dead, I'm afraid.