Your third pet will be hit by Pluie acide, even in the back row, so if you don't want it to die, I suggest having at least 150 HP on it.
There are ample opportunities to power level a third slot using this strategy once Stickers is out. I'll leave it up to you to decide when is best to swap it in, but it's probably going to be a round that Baiser de grenouille is used.
Note: Please keep in mind that to complete the Achievement, all 3 pets must be level 25. Leveling options are included for alternate completion methods, but will not grant credit towards the individual or meta Achievements.