Input Clock Multiplexer for PLL (CIII)

The only way I know of to mux 3 clocks at the input of a PLL in a Cyclone III without using any extra external connections as pletz has suggested is to use two PLLs. By passing one of the clocks through an extra PLL, you can then use the clock control block to switch between all 3 clocks. This method has some drawbacks such as wasting a PLL you may need for something else and increased jitter when using the clock that ends up going through two PLLs.