As far as your question:
" the audio track i use on Sonar for the guitar, the vol slider will not control the volume of the guitar, only the vol controls of the x3 will adjust vol"
Thats is all part of standard gain staging which I explained on how I set mine up.
When recording, what you have set up as the signal coming in is what you have and the slider should have no effect on what you see being captured as you record.
As far as being able to hear the mix the way you need to to lay down your X3 guitar track, adjust those sliders for the tracks already laid down to where your happy with what you can hear but MAKE SURE to set the X3 tracks levels as I explained for the track you will be recording.
I was lost for a minute trying to figure out what you were getting at in the original post but I understand now. I think what you have setup is correct, just the understanding of what Sonar is actually doing might have you confused a little bit
Just follow what I put out there for ya and that will get you going.
It sounds like you need to get a better understanding of setting up for recording initially as far as gain stage, all the levels and routing, and the basic processes of recording with/when other tracks are playing Then the mixing comes once you have caught all your tracks, thats when you will worry about the faders.
Please dont take this statement wrong, I dont mean any disrespect by it. Just kind of what Im thinking to help you get there and understand the initial setup and what is actually going on
Basically I think you can IGNORE all my other posts LOL but this and the one where I explain how I set up for recording with mine. Sorry for so many edits, I was confusing myself
Others might chime in with a better/easier way to explain it
You have great equipment so I wouldnt question that at all
Ill check back later to see how your coming, please keep us posted