how do i link the same gear into 2 slots and a default glamour?

[Callinon]

my 3 casters share gear. my smn take the glamour 1 look, my rdm takes glamour 3 look, while i want my blm to keep default gear look

but the glamour from smn kept overwrtign the blm/rdm one. what gives?

So the glamour system attaches the look of one object to another object. That's all it does, even when you're using plates. This means that your SMN and RDM glamours are overwriting each other each time they're applied. Your BLM is just out of luck here because there's no way to create a glamour plate that dispels glamours. The best you could really do there is to create a third plate that just carries the default look of the gear set you're talking about.