Can one locus have three different alleles with one dominant and two recessive? Can there be more than three forms of one allele at a particular locus?

I know that the ABO blood system has three different alleles at the locus for ABO blood type, and that IA & IB are co-dominant to i, but if a particular locus has three different alleles (arbiltrarilly represented by K, k1 and k2), can K be completely dominant over k1 and k2? If a (sexually reproducing) organism inherits both k1 and k2, but K, could both k1 and k2 be co-expressed? (assuming the normal somatic number is 2n) Could k1 be completely dominant over k2, but still recessive to K? Are there any examples of this happening? Can another epistatic gene make one recessive form be expressed over the other? Could there be more than three alleles at the same locus?

Study Genetics.