NOTE: each yarn is used on one row at a time, and you only turn your work after 3 consecutive rows are worked in same direction, one with each color. The interesting part of this is although you are only knitting, because you are forcing the direction of the work, garter stitch is not what you create, but rather alternating horizontal stripes of stockinette and reverse stockinette. Pretty interesting, considering no purling required!
Do not fasten off after every row, carry all yarns between use.