Hey SD,
I have not upgraded to X3 yet, BUT, I suspect it would since they work the same way.
If you want, ask "Rich Kahn" (tech support guy for OneCNC america, FL), he can define the differences for you if they are different.
Just try it out and see if it will post to mach3 and mach 3 run it. Also, in OneCNC you can edit your on Post.
scott