Casting On with 3 Different Yarns

I am a rooky knitter. Want to make a purse with three “panels” of different color yarn. Want to cast on 15 stitches of each color for a total of 45 stitches. How do I connect the colors while casting on so I don’t start out with three separate sections. I know how to change colors while knitting without creating holes, but don’t know how to cast on so I end up with one long row of stitches instead of three sections of 15 stitches of each color.

I would use a knitted cast on, or cable cast-on.