Writing a subroutine that activates another subroutine....

Ok so basically here's what I want to do..

I have 2 addresses that I want to be at a certain value before a code activates. All 3 are undma'd addresses.

So in essence I want it to make it so that when Address A reaches a certain value it activates Address B which in turn makes code C work.

I know how to make one address activate a code and how to make 2 addresses activate one code (somewhat) but I'm pretty stumped on how to get this to work.