McCall's sewing pattern books use to have a kids three musketeers costume. My mom made them for my 3 little nephews when they where 6, 4, and 2. They were a HUGE hit.
My 4 year old wants to be a Mickey Musketeer for Halloween this year. I have a red long sleeve t-shirt and some khaki pants that I'm going to shorten into pantaloons. Mimi will sew the blue/gold vest thingy with the white collar.
McCall's Pattern 7282
It's a few years old so the numbers may have changed.