dict attack with GPU

i try to crack a truecrypt file that i know the password is a 3 word from a big list (16000 word (dict.dict)) and i have a 3 GPU

i write a command:
cudahashcat64.exe -a 7 -m 6211 c:\file.tc ?a?a?a dict.dict

the hachcat start and the program run only in 1 GPU. and the other GPU dont work.
how can i run in the other GPU to?