cant rember the nock hight set for shooting 3 under.[]

Depends on the tiller. Only way to find out is to tiller. I have two bows tillered for 3 under and one is 1/4 high over center, and the other a little higher. And other bows that are tillered for split, shooting 3 under, they are 1/2" high over center.

Get some bareshafts or broadhead tune to adjust.

Pretty much what BC said. Start with a nock point like you would shooting split and adjust it from there. It'll probably tune to be a touch higher. How much can vary from bow to bow and shooter to shooter.