There are hundreds of patterns that use just 3 colors. Easiest out there are split rail, nine patch or a churn dash. If you can use various shades of the three colors, then log cabins would work. I think the better question is what style does she like? Is she traditional, 'old-fashioned' or more of a modern person? If she is modern, then an illusions quilt using white as the background, grey for the 'shadows' and a variety of yellows for the blocks would be outstanding!