Dear Bryant,
If you don’t have a wire connector, you could try the following:
If the two white common wires are thin enough, you may be able to insert both into one C slot.If the slots are too narrow for two wires, use a wire connector (available at Amazon, Home Depot, etc.) to combine the two common wires into a single strand, which can then fit into one C slot. The third common wire can be connected to the other C slot.
Thanks,
Netro Support