Q: What is forbidden in the third commandment?
A: The third commandment forbiddeth all profaning or abusing of any thing whereby God maketh himself known.

Malachi 2:2. If ye will not hear, and if ye will not lay it to heart, to give glory unto my name, saith the LORD of hosts, I will even send a curse upon you, and I will curse your blessings: yea, I have cursed them already, because ye do not lay it to heart.
Isaiah 5:12. And the harp, and the viol, the tabret, and pipe, and wine, are in their feasts: but they regard not the work of the LORD, neither consider the operation of his hands.

Westminster Project - Thomas Watson's Body of Divinity on the 2nd Commandment