There are many backing track web sites where you can get for free or buy the backing tracks you want , you firstly need to know how to download these files and save them in to the ST3 folder.
Depends on songs you want, just type in the name of the song in google and place backing tracks, or karaoke etc this will give you lots of options , youtube is also a great source of backing track but depends on which songs you need.
I would start with stereo files in .m4a format which ST3 likes , once you have learned how to download the files you need to learn how to put them in to ST3 folder, this can be done in a few ways , your downloaded files will be in your Download folder on your device , simply select the files and COPY them ,
Then find your ST3 folder which should be found by opening your FILES FOLDER on your ipad , there you will See Stage Traxx 3 folder just open it and save the copied downloaded files, now open up ST3 and select SONG mode , then pull down on that screen which will resync your data files and now you should see all the songs you have placed in to the ST3 folder.
There are other ways but i would stick to one method first till you get confourtable with using ST3 because there is a lot to learn , but once you get it in your head you will see the logic , which has been designed to make it very easy and flexible to use on stage.
Keep or rockn, Damir