3-way switch w/ dimmer
You didn't specify the Lutron Model number of the switch but most of the Lutron Maestro switches are not like conventional 3-way/4-way switches. They require a Lutron companion switch at the other 3-way location in order to operate properly. Check the instructions for multiple location installations. Make sure the circuits off when you change the other one,  hooking them up live can kill them, and they're not cheap.