Hi, and welcome to pet battling.
My advice is - don't think in terms of "a team". You should never, either in PvE or PvP, call something "your team".
In both PvE and PvP, you need to keep re-combining pets into different
teams for different
battles. Since certain pets are strong against others, your objective will be to line up pets that are strong against whatever the opposition team is. You choose a different team for each battle.
Have a look at http://www.wow-petguide.com/
to get a sense of some of the different teams you will need to handle different PvE tamers.
Have a look at http://pvppetbattles.com/guides/creating-a-team/
to get a sense of how to choose pets for a PvP team.
You have got 3 pets to level 25. Your next task is to get a few more pets to level 25, develop your roster so that you can start beating the tamers and levelling more. Your next pets to level should be the ones that will most help you level other pets - you want to be able to form the teams that beat the tamers.
Which pets should you develop? As you work through the tamers you will see some pets come in handy again and again. You have a Blue P/P Emerald Proto-Whelp
and a Blue P/P Nexus Whelpling
at level 19. Both of those will be very useful. You don't have an Anubisath Idol
yet; it's worth your while to run the Temple of Ahn'Qiraj and kill Ver'lor and Vek'nilash http://www.wowhead.com/npc=15276/emperor-veklor
each week to get one - or you can buy one on the Auction House.
You have followed the Tamer trail up through Taming Outland, though I don't think you've got past Bloodknight Antari. If you go to his Wowhead page http://www.wowhead.com/npc=66557/bloodk ... i#comments
you will see lots of teams that can be used to beat him.
Then carry on through Northrend, Cata, and finally Pandaria. By the time you've finished Pandaria, you will have added more families and useful pets to your roster at level 25.
Meanwhile you can try the Draenor Tamers, which don't require you to have done the earlier expansions. http://www.wow-petguide.com/index.php?m=5