It's pretty weird-looking, but this works:
setAttr(cmds.polyCube(name="Bob")[0]+'.t', (10,5,-6))
I'm not sure what your real requirement is, but to get it on one line I'd be happier using a semicolon:
bob = cmds.polyCube(name="Bob")[0]; cmds.setAttr(bob+'.t', 10,5,7)
(I'm using cmds instead of mc, but I think it's the same thing)