If you want to make sure your ribbing is joined from the get-go...
Needle #3 should have the length of yarn you will work with at the end of that needle (closest to your left hand). Take the last stitch from needle #1 and move it to needle #3; then take what was stitch #1 on needle #3 and move it over that stitch to be the first stitch on needle #1. Start knitting with the stitch that now has your length of yarn. It may take more than one try before you can do it easily, but well worth the effort for a very nice join.