The simplest way is to draw 3 boxes. You'll have to position each one on your own doing your own math to determine where to start the first one to make it centered etc.
First position your cursor at the top left of the first box, draw it, move to the top left of the next box, draw it, and do the same for the last. Here is some code:
<esc>&u300D<esc>*t300R<esc>*p300x300Y<esc>*r3U<esc>*v2S<esc>*c300a300b5P<esc>*p600x300Y<esc>*r3U<esc>*v3S<esc>*c300a300b5P<esc>*p900x300Y<esc>*r3U<esc>*v1S<esc>*c300a300b5P
Here is the explanation:
<esc>&u300D<esc>*t300R -- set the Unit of Measure and Resolution (in this case 300 dpi)
<esc>*p300x300Y -- move cursor to 300x 300y (1 inch x 1 inch)
<esc>*r3U<esc>*v2S -- set the color palette to RGB and use color 2 (green)
<esc>*c300a300b5P -- draw a box that is 300 wide and 300 tall, use current fill pattern
<esc>*p600x300Y -- move cursor to 600x 300y
<esc>*r3U<esc>*v3S -- set the color palette to RGB use color 3 (yellow)
<esc>*c300a300b5P -- draw a box that is 300 wide and 300 tall, use current fill pattern
<esc>*p900x300Y -- move cursor to 900x 300y
<esc>*r3U<esc>*v1S -- set the color palette to RGB use color 1 (red)
<esc>*c300a300b5P -- draw a box that is 300 wide and 300 tall, use current fill pattern
Here are the other colors and palettes, keep in mind this is the simple way, you can specify your own RGB etc.
RGB Palette
<esc>*r3U<esc>*v1S - Red
<esc>*r3U<esc>*v2S - Green
<esc>*r3U<esc>*v3S - Yellow
<esc>*r3U<esc>*v4S - Blue
<esc>*r3U<esc>*v5S - Magenta
<esc>*r3U<esc>*v6S - Cyan
CMYK Palette
<esc>*r-3U<esc>*v1S - Cyan
<esc>*r-3U<esc>*v2S - Magenta
<esc>*r-3U<esc>*v3S - Blue
<esc>*r-3U<esc>*v4S - Yellow
<esc>*r-3U<esc>*v5S - Green
<esc>*r-3U<esc>*v6S - Red
<esc>*r-3U<esc>*v7S - Black