On selfing a plant of F1 generation with genotype AABbCC, the phenotypic ratio in F2 generation will be:
3 : 1
Since only one gene is in heterozygous condition, only two types of gametes will be produced by either parent. The gametes will be ABC and AbC. The cross of them will produce three combinations as follows. For two combinations since one dominant allele (B) is present, it will produce the same phenotype whereas the homozygous recessive condition f that gene will give another phenotype, as shown in the following figure.
Since only one gene is heterozygous, the ratio of two phenotypes will be similar to that of the F2 generation in a monohybrid cross i.e 3 : 1