Which Triple did you get? 2,3 channel, or one of the new 3 channel multi-watt versions?
I own a 3 channel Dual, which is the same amp minus the additional 3 tubes for the additional 50 watts of output power.
When the manual says not to max the gain out on channel 3, that is only a suggestion. You really don't need anything past 3:00 as it just starts to fizz out and creates a mess. You still can utilize it though if you set the rest of the channel controls properly. On channels 2 & 3 the less gain you use the tighter the amp will sound at volume. Believe it or not the Dual and Triples are not the distortion producers they are made out to be. The problem is most haven't really figured out how to work the tone controls.
The best response you will get from your amp is if you set the effects loop to either ON, set to the channel you are using, or to activate via footswitch. This allows you to utilize the Output control to the left of the channel controls. Pick your channel you want to use and run the channel volume to around 11:00-1:00, run the rest of the channel controls straight up at 12:00. This will give you a good starting point.
If you read the manual regarding the tone controls you may have picked up on the statement where it talks about the signal path running through the treble control first. then to bass and mids.
The more treble you use the less the bass and mid knobs work.
So dial back on the treble until you get good response from the bass and mid knobs. Set those how you like them and you can use the presence knob to dial back in some of the highs you lost by turning down the treble.
What I do on my amp is then increase the gain until the only thing I can hear change is how much the gain is compressing then back it off until the extra compressing of the "dirt" stops. That is the "distortion" sweet spot for your amp using the tubes in it at this time.
As to your idea regarding a gain pedal. Yes, a gain pedal will enhance the tonal and gain structure of the amp. Most use a Tube Screamer type pedal in from of a Rectifier to "tame" some of the low end these amps have. What it does is tighten the amps low end. To get this they use the pedal as a boost, running the pedal gain at full minumum and maxing the output of the pedal, tone control, if any, is set to player preference. The Mesa Grid Slammer is Mesa's version of a TS if memory serves right. My personal favorite pedal for my music is actually a Metal Zone with gain all the way down, output maxed, and the tone controls all set to about 10:00 on the pedal. I feel all this actually does is gives me a mid bump a little higher than where it is on the amp stock.
What cabinet are you running the head into? I ask because I recently found out while using my bass player's Marshall Cabinet at his place (JCM900 1960A) was that my amp would actually cause the cabinet itself to violently vibrate when holding out chords at rehearsal/stage volume. It does not do this on my Mesa cabinet, nor with his Marshall head on his cabinet.
Keep in mind that the Rectifiers tone stack sets the bass where it feels like it starts about 2 feet below ground surface and continues up til about head high with the treble. On another amp, such as a Marshall, the bass starts about where the cabinet begins and rises up about two feet above your head with the high end. This is my perception. I'm telling you this because if you remember that explanation, you set your tone up following this mental image, and you wont run into the problems that cause people to bag on these amps. Think of a Rectifier as a weapon that causes the earth to rip and cause a soil tidal wave at the listener, where another amp does damage, but not at the same catastrophic manner to the environmental surroundings.
Last thought. Make sure you acquaint yourself with the rest of the channels and modes besides 3 Modern cause it sounds the most metal right now. I do not use Channel 3 any longer except for leadwork and I use 3 Vintage for that. My main rhythm is Channel 2 Vintage using tube rectification.
Also make sure the loop knobs are set properly, IE. the send knob is at least @ 12:00 and the Mix knob is at least at 3:00(I set the mix @ full).
To get you started here are a couple settings:
Pat O'Brien on his Triple Red Channel(on a 3 channel, this is Channel 2 Vintage) Diode Rec, Bold power, Presence: 1:00, Gain: full, Bass and Treble 1:30, Mids: 10:30 He uses a Two channel Triple so if you have a 3 channel it will only get you in the ball park.
My rhythm setting Channel 2 Vintage, Tube rec, Bold power, Presence 12:30, Volume 1:30, Gain: 3:00, Bass 12:00, Mids: 2:30, Treble:10:30, Output set to hear myself over band, but never less than 9:00 when not with a band. The amp doesn't open up otherwise.
Clean tone Channel 1 clean, Volume 12:00, Gain: 11:00, Treble: 12:00, Mids:2:00, Bass 11:00, Presence: enough to give sparkle to output, but not enough to make it too bright.
Rectifier simulating an old Marshall(think somewhere in the JTM 45 area) Channel 1, pushed, Tube rec, Spongy, Volume 1:30, gain: 3:00, Mids 3:00, treble: about 2:00-3:00, bass about 9:00-11:00, Presence: to add crispness(usually not above 11:00)
Good Luck with your amp man.