Triple MPD is such a trolly team. Hate them with a vengeance, even a single one. I'm not sure I have something exceptionally good against three of them, but here are a few pets that I like to use against them.
S/S Fiendish Imp. Burn hits hard, Gate is faster and most importantly Immolation. Immolation from back line helps to burn through Decoys and is of course also strong against the MPD, and you get to heal up a bit with human racial. Other pets can of course also be good for this, but speed is king agains these annoying pets. Just watch out if you try Minfernal or Netherspace Abyssal for Immolation, the MPD breath attack is strong against these magical creatures.
S/S Prairie Mouse. Flurry(on any pet faster than 309 speed this will eat through a decoy). But my current favourite move is Poison Fang and then Stampede. If your opponent does the usual Decoy and then AoE, you will first have poisoned them(strong dot vs mechs) and then stampede will burn through the Decoy and the third hit will leave shattered defence up allowing 3 dot ticks to do double damage, this will bring them very low even if they switch away. Critters also take weak damage from the AoE.
Other pets that I find useful are pets that have things like Nuts/Eggs/Make it Rain.
Another one I like, and soon it is summer for those who don't have it, is the S/S Qiraji Guardling. 325 speed stun, hits pretty hard when it lands, and Sandstorm to apply some kind of team defence against the AoE.
Cyclone. S/S Qiraji Guardling does really well against the Dragonkin cyclone pets. Strong crushes and Sandstorm pretty much negates the Cyclone. Also I love to use the stun against the Jademist Dancer just after they use Rain Dance, they won't get their big hits.
Voodoo Figurine, or anyone with Sear Magic kind of ability easily removes the cyclone of course. The figurine have the benefit of being able to dot the pet casting cyclone so they will still be damaged when they, like they often do, switch out.