In the math node, there's a third value slot. If I'm copying a shader from blender, there isn't a third value. Do I put a zero in that or leave it at .5? Either way, depending on the function, it seems like it could change the output.

I contacted tech support about this some time ago inquiring about this same concern.  They responded that as long as you don't use the value 3 the node will work as if you are using 2 values.  But I do things on the side of caution, so I would zero it if you are doing adding and subtraction, and set it to one if doing multiplication or division.  I'm not that trusting.

Blender does have a third input and the one case I found for it is Multiply vales 1 & 2 then add in value 3.  Also the Poser Documentation should have listed all the cases where the third value is useful.  That third value threw me the first time I saw it as I was trying to copy a shader and it had a math function with only two inputs and all i could find was 3.

What version of blender are you using? I'm using 2.93 and this is the math node I get

Or maybe I should ask which node you're looking at?

Try changing the type to multiple add and it should get the third value.  I don't use Blender just that When I had the problem it was with a cycles material and I looked up the third value and found it mentioned in the Blender site.

Poser doesn't have the ability to change the number of inputs on the fly (unlike Blender). So if an input will ever be needed, it will always show up, even if made irrelevant by the other settings. Sometimes that's confusing.

Yeah, I've started to notice that. In Blender, the 3rd value doesn't show up often, but some of the other nodes do it more.

You can use the standard Poser math node in your Cycles setup, especially if you are concerned about accuracy in the calculations.  It will accomplish the same results.