How long to bake a pre-baked ham?
Pre-baked hams just need to be warmed through.  You can augment it with a glaze or other flavorings, but really it just needs to be warmed up.

But 3oz seems really, really small.  If it were larger, I'd possibly heat it up covered so that it doesn't dry out before it warms through in the oven.

If it's really 3oz, I'd assume it's a ham steak, and you can just cook it in a pan on the stovetop 'til it has a little bit of color on each side.