A flower's petal color is determined by three genes, R, G, and B, with two alleles each. The R gene has three genotypes: RR = 5 color units, Rr = 3 color units, and rr = 1 color unit. The G and B genes have similar effects. A plant with genotype RRGGBB has 15 color units, while rrggbb has 3 color units. Two trihybrid plants, RrGgBb, are mated. What is the expected proportion of progeny plants displaying 15 units of color?