That is completely normal for a 3 year old. One of my sons used to nuzzle in on his babysitter's chest. It wasn't something he did with me though. She was a large, well-endowed woman. We laughed about it, deciding he found the unique experience of cuddling with a women with large boobs comfortable and cushy (large boobs were not something he experienced with me by any means).

If it bothers you, just gently move him off of you or to the side. That would be an appropriate correction.