I've recorded a track who has no head room & I would like to go to that track & raise 3db.
... I've got the mixer all the way up & I don't want to redo the track.
I do not understand 'headroom' in this context but let's try:
You have recorded to one track and you pushed its volume fader fully up. You do not have too much level at the output (no red light) and you do not want to change the overall volume but want 3 dB more on this single track. There is no plugin in the channelstrip of this track. Are all of these statements correct?
If yes, do the following:
Pull the volume fader of this track from its top position (+6 dB) down to -6 dB. Insert the plugin 'Gain' in the first plugin slot. You find it under 'Utility' in the plugin list. Set the gain of this plugin to +12 dB. Now the track is as loud as before but you got plenty of room to make it louder using the volume fader.
If you have a different situation, then tell us about it. How many tracks, how is the level on the other tracks, where are the volume faders, how is the output volume and where is the volume fader of the output. Posting a screenshot of the mixer would help.
And of course, tell us about your target, what for are you doing this mix? Is it even music and which genre? A video track with a bunch of humming insects requires another approach than a death metal piece where one guitar is too quiet. The techniques are very different.