Using common hanging basket/ bedding plants, how about trailing lobelia in
blue and white alternately around the edge, and red salvia in the centre

I also did a quick search on alta-vista:

+"hanging basket" +red +white +blue

which suggested:

http://www.patioplus.co.uk/rec5b.htm
( Red Verbena Tukana, White Bacopa Cabana (a large flowered Bacopa) and my
old favourite - Scaveola Blue Shamrock, plus a Red and White Fuchsia -
Snowcap)