Barbarian fly fishing for best xp.
You could be talking about fly fishing at barbarian village(best for f2p) or you could be talking about the actual barbarian(heavyrod) fishing. Hopefully you're talking about the village(just fish xp) as heavyrod fishing gives str xp in addition to fishing xp.
It would be pretty hard to do shilo quest on a level 3, so I'd probably just stick to barb village fly fishing. It will take a while but it works.
Btw, if I remember right, you can get a friend to help you with bosses during Shilo Village quest(with recoils, etc) so you can fly fish there. If you manage to get that done, that would be the absolute best for a level 3. You should lose barely any money...unless of course you are looking to make money. If you want to do that, do sharks, etc as others have suggested.

