Its very common, trust me. Its usually just seeing how much he can get away with.
If he does it to you while holding him, put him down & tell him NO scratching hurts & walk away from him.

If he does it to your 3yr old, you need to do the same, tell him no & turn your back to him & ask if your 3yr is ok. Avoid too much attention going to the behaviour. Negative or positive it is still attention that he will learn the scratching gets. So best to just say No firmly & then either walk away or turn your attention to your 3yr old & ask he is ok etc.

If it gets too much then I don't see a problem in taking his hands & placing them by his side while saying NO SCRATCHING & maybe even taking him to sit by the wall or something.

Good luck with it. Its one of those things that they do eventually stop doing though.