How to calculate input and output capacitance in RC model

Hi, I was just stuck in calculating the input and output capacitance in RC model for CMOS. I can understand a unit inverter has input and output capacitances as 3C while the PMOS has twice the unit width and the equivalent resistance is R. But for a 3-input NAND and a 3-input NOR with the same equivalence resistance (that is R), the input capacitances are 5C and 7C while the output capacitances are both 9C. I've no idea how to calculate these. Can anybody help me to figure this out? Thanks.