DS1 is 3yo next month and has just started testing boundaries in recent weeks. Nothing too extreme just a lot of demanding and shouting NO and refusing to do things, not listening, testing how rough he can be with his little brother... A bit of throwing and kicking toys, not when he's upset but more let's see what happens if I do this.
To be fair he's mostly just bit annoying - putting his fingers up your nose while trying to read at bedtime, major delaying tactics at bath time, when leaving house etc - rather than naughty and sometimes it's hard to distinguish when you're feeling tired and irritable and frustrated etc. I know most of the time he just wants attention and a good dose of ignoring and distraction works but I think it's changing a bit now where we need to do a little more. I don't think I can avoid 'proper' parenting much longer 
So I'm thinking it's probably a good time for the odd sticker chart/reward jar etc.
Shat have you found works for you?
What behaviour do you ignore and what is a non negotiable for you?