If I understand your numbering system, B1 is the bottom three pins, B0 above that, then B2, with B3 on the top.

I expect to take 3.3v off of B3, as the connector that I have that fits that pin spacing won’t easily fit the inner connectors.

To check that I had the relay pins correct, I tried to check by hooking it up to B1, since I know B1 works, as the pen lift operates properly.

Not knowing the polarity ahead of time, I metered the pins and was surprised to find +5V center pin to the rearmost pin and -5v from the center pin to the frontmost pin. I was expecting 3v.

If my 3.3v relay will be happy with 5v, then no problem. I figured I should check first. Am I reading this wrong? 

And, assuming I’ll be using B3, which pins should I use to trigger my relay?