First lets start with levelling 3 pets. The crucial thing here is to maximize the XP you gain. The easiest way to do this is to battle pets that are 2-3 levels higher. Any higher and you are probably going to risk getting killed and you will need to heal/resurrect pets too often.
Another thing to bear in mind is to pick your battles if possible - try to find an area where the pets have a weakness against one or more of your pets. This means quicker kills and less damage taken. This might help squeeze that extra pet battle in before using your healing ability!
Finally lets consider levelling low-mid level pets. With the exception of wild pets, many of you will find yourself faced with a mind-boggling grind to get your favourite companions up to a high level. But fear not there is something you can do to help!
Choose whichever pet you want to level and then put it in your party along with 2 high level pets. Now you simply go and battle pets at the level of your higher level pets! Obviously it is best to start with the low level pet as swapping them in will result in extra damage with the lost round. To get that little bit extra experience you should choose your battle, as I said before, and try to only use 1 of your high level pets to win the battle. This means the XP is split over less pets, giving your low level pet more XP! Also another tip is if your ability to swap pets is removed, it may be best to forfeit the battle. By doing this you cut your losses and avoid having to resurrect or heal your pets.
If you are saavy about which high level pet you use, you will hopefully have no problem with the 8 minute CD on Revive Battle Pets. However when the unthinkable happens and your high level pet dies whilst you are still on CD, it is not necessarily the end of the world. Remember you have 3 pet slots! Just switch to your other high level pet until the CD is over. Simple.
Bear in mind if the pet is level 1 and you choose to battle a level 24 pet you will probably get one-shotted. You may need to pick slightly lower level battles until your pet is level 5 or so.
Some of you may wonder about power levelling more than one low level pet. This can be done as long as your high level pets strengths are played to. This could be the damage bonus added by pet weaknesses or a healing ability, it doesn't matter. What is crucially important here is that you have a very strong high level pet as you will not have that back-up pet should things not go your way. One thing you will notice is that by having two level pets your overall XP for each individual pet will be less due to the split. Personally I prefer maximizing the XP gains and focusing on a single pet but the option to do two is there.
That's all the advice I have! In the end of the day it comes down to maximizing your XP gains by any means possible and working around your 8 min CD for resurrecting pets. Quite simple really.
Hopefully some of you will find this guide useful and informative. Feel free to ask questions or give any extra tips you may have!