Skid thats why you have an input trim on external mixers so you use them to fine tune the sources and then use your channel faders for final mix , as long as all your tracks are recorded reasonably consistent in level you should not need any tweaking on the night on ST3 if you have an external mixer, as thats your logical tweaking spot.
It all starts in the prep work if thats done correctly hardly any tweaking should be needed along the way.
Its a different story if all your multitracks are all over the place in level then you are allways chasing your tail through the gig. and yes it would help to be able to retweak all your mixes as you perform and for them to be remembered, but thats dangerous and laborious to do on the night it should be done in prep.
Thats assuming your external mixer is near you and you can tweak it on the run.