So on a mach 3 board you have I3 to hook up Homing switches. I2 is for limit switches. There is a little overlap here since you can do 6 limit switches in series for X, Y, Z. I am assuming you wouldn't do limit switches and home switches.

So, is doing 3 and 3 the right way? Do XYZ home switches on I3. Then do XY limit switches on the other side of travel. Probably won't do a z bottom switch, so that is 5 total.