Re: G3 type tones-are they just good tube amps with pedals in front of them?
Satriani's older tones... were going through a Marshall. At least that's what the man himself says and shows us in the Special Features on his 'Live in San Francisco' DVD.
He used the clean sound on those Marshall's, and then beefed it up with a modded Boss DS-1 pedal. That's basic Satriani Clean/Dist tone nailed. The rest of the tones that were pretty much created with a chorus pedal, delay pedal, and wah.
If you wanted his tone, particularly that on the DVD... i'd use a Floyd Rose'd guitar, with either a Pearly Gates Bridge/Pearly Gates neck combo (as he used on one of his guitars for the DVD), or the same guitar with a Dimarzio PAF JOE/FRED combo... i'd put that through the clean channel of any decent marshall tube amp (doesn't really matter which, you just need the basic tone.). Then get a modded DS-1 pedal from Robert Keely. 
And your done for his basic tone.
Steve Vai's and John Petrucci's tone is a lot more complex involving much more rack gear.