clock pin assignments and cyclone III

What's the error message? I/O banks are programmable, i.e. they can be configured to support different voltages, but all I/O in that bank must use the same voltage reference. Usually no-fits caused by this are because other pins in the same bank require a different voltage. The no-fit error usually expands to to show where the conflict is.

Another place to look is the Pin Planner. You can use it to list all I/O in that bank and look at their voltage. You can also run an I/O analyzer to do quick checks, rather than running a full compile and waiting for it to error out in the fitter.