3" long straight cutting router bit (like this one on Amazon). Make a good jig for the router to follow, and make multiple passes so you're not trying to chew through too much wood at one time. 3" bit will get you half way through, then flip lumber and jig over to go rest of way through. A sander will take care of any ridge if you were slightly off with the jig. The jig will also assure all three pieces look identical.