Making a model with about 10 LEDs' what resistance to I need?

. Sorry I cant remember the Voltage of the Led's.... but small ones and there's about 10 of them and the transformer is at 3V..... They are wired in parallel and it's too complicated to add a resistance to each LED... so last time the guy in the shop gave me ONE resistance to stick at the end of the connections just before I connect it to the Transfo.... but now I cant remember what Value it was. Sorry to be so vague about all of this but my last model is all sealed up so I cant get at the wiring to see what I did before.