Hi finished recording an instrumental song.
...Do I need to creat a group channel to place all three tracks into it and then from there master the group channel on a whole? Any suggestions on what type of theshold I should have the mastering on?
Not sure what you meant by making a 'group channel'. You should already have a master output –a 'group/bus, with final fader, meters, trim likely, a pan, effects insert point? ...If you happen to be skipping all that and your tracks are going directly to the stereo hardware outs?.. Then yes. Get it in there.
(Umm in that case in the future.. start with it in there
Mastering typically means in a separate process the assembling, balancing in various ways multiple song tracks.
Given a single song -but particularly as you are doing the mix, what are you really doing but switching attitude a bit –and further refining your project?
There are a few ways to go here, and part of it can be the head prep' of getting away from the mix process- time away from it, and/or as well as the act of setting it up as a new 'mastering project.
But. You may just as well, with this 'seeing it anew', and with more options for pursuing this
improvement you're now making right? ..continue refining it in the mix project.
You can use this same approach –refining in the mix projects, in the case of multiple songs headed to a common group of songs as well, all be it a little less efficient than assembled under one roof.
Last, when you now decide to compress this- you are running smack into a question; In which and in what ways are you going to make this thing sound better? What does it need? 'Hearing' that, and some experimentation, leads you to what you do there.