Congrats!
the Mk III is a underapriciated & very kickass amp. I have an '85 no-stripe simulclass long head that I either play through a Marshall 1960AV 4-12 cab or a Mesa 1-12 half open back long cab with a C90.
Is your Mk III a Simulclass, 60/100, or a 60 watter? If it is a simul-class, I recommend putting EL-34's in the outside sockets, if you don't have them in there already. That will really round out & even out your tone.
Using 15" ported cabs may make your tone fart out & sound too flubbery....15's, IMHO, are for bass, NOT guitar. Your mids will get lost using 15's, meaning you'll get lost in the mix and won't be able to cut through if you're playing with a band. I recommend getting a good closed back 4-12 or 2-12 cabinet with some good speakers. I like Celestion vintage 30's, but C90's are good too. Let your guide you on that.
If your amp has the onboard 5-band eq, it's really pointless to put a eq pedal in the effects loop. I do not recommend putting a Boss GE-7 in the effects loop. My experience is with that pedal is that it takes the warmth & depth away from your tone when put in an effects loop. You might as well sell your MkIII & buy an old solid state Crate & go to the strip club with the money you have left over if you have to put a GE-7 in the loop, because thats what that pedal will make your amp sound like!
Assuming your amp has the 5-band eq, you may try these settings on the lead channel as a starting point for a good metal tone, and fine tune it abit to fit your speakers, guitar/pickups, & your personal tonal preferences: Volume at about 5-8 (Bright pushed in), Bass around 5 (shift pulled out), Mid at 9-10(Rhythm 2 pulled out), Treble at 7-8(shift pulled out), Presence between 2-4, Lead Drive between 8-10 (pulled out, of course), and set your master volume & Lead master volume (with bright pushed in) to your desired volume level, if your using single coils, pull the Deep on on the Master Volume, if using humbuckers, keep the Deep pushed in, run at full power, with the 5-band eq in the "V" formation. This should get you into "Master of Puppets" territory. A good quality compressor pedal may be a good thing to put into the front of the amp, for solos. A BBE Sonic Maximizer might help you fine tune your tone also, but that may not be nessesary.
Have fun with your new toy!