1) The CRP550 board. This board is just a breakout board for the motor outputs that simplifies wiring, and is run off of port 1 on the Smoothstepper in your system. It is mapped as follows:
Pin Function (re-assignable, but this is how our XML is set up)
2 X Step
3 X Dir
4 Y Step
5 Y Dir
6 Z Step
7 Z Dir
8 B Step
9 B Dir
14 A Step
16 A Dir
This card also conveniently passes through all of the IO on the second ribbon cable header. The ESS has enough current to drive most isolated outputs directly, so you could break out IO from this second ribbon cable header on the CRP550 board and use pins 14 and 16 for the STEP and DIR functions, and either pin 1 or 17 for your enable.
2) Port 3 on the Smoothstepper. This port is unused and is open for whatever functions you would like to use. This would be the same approach as using the pass-through on the CRP550 board, but you could do the use any of the output pins (1, 2-9, 14, 16, or 17) for your outputs.
In either case, probably the best way to physically access this IO is to get a small basic (un-powered) breakout card that just gives you access to all 25 pins. Below is a picture of a system we did recently with a little BOB like this that we used to drive an auxiliary lift table -- I've circled the cheapo Bob and the passthrough port on the CRP550 board in green. Dgage, we used the same BOB in your system to pipe 3 outputs over to your auxiliary relay box. We bought a small lot of these little things for odd jobs like this and can offer them for sale, or you can use any basic DB-25 breakout board and a DB25 to LPH-26 ribbon cable. If you needed to use the inputs on port 1, you'd probably want something more sophisticated than this, but for just 3 outputs this should be fine.
Ahren
CNCRouterParts