I ran a very successful 3 mech team a while back, a Bleed team variant with Pierre, Anodized Robo Cub and Liflike Toad. Took advantage of Rebuild, heal mechanicals in your team. But I don't think this team would do very well against 3 opposing mechanicals.
A quick smart search for family = mech, strong vs mech and you get a list of 8 pets.
Clockwork Gnome is probably one of the easiest with Railgun, a heal with Repair, and Turrets to handle Decoys.
I would guess the strong AoE attack of Thunderbolt from Darkmoon Zeppelin and Mechanical Pandaren Dragonling would be VERY useful. Best bet is probalby the Dragonling since you don't need to trade away the Decoy for the AoE attack.
Blackfuse Bombling has one attack that will hit really really hard, but it also has a long cooldown.
Pierre with Heat up could be interesting if you face multihit Lil'Bling, but ultimately I don't think Pierre would be up to the challenge as a whole.
Pocket Reaver and Sunreaver Micro-Sentry both have Fel Immolate which hurts pretty good. But if chosing between them I would say Sunreaver wins out with Extra Plating.
Tranquil Mechanical Yeti only has Blizzard that is strong vs other mechs, does not sound like an ideal pet for this challenge.
Rebuild might be useful for this challenge, check out the chickens or the robo cub to see if there is anything you like there.