use

使う(つかう)	Use. $$$

用いる(もちいる)	Means the same thing as 使う, but more formal. As in, “Please use this pencil to complete the test.”

遣(つかう)	This is very specific. It means “to dispatch an envoy”. Or, to be careful or give consideration to. Usually only used with the phrase 気を遣う, but even then it’s usually hiragana so who cares. Good luck ever using that.