The two co-dominant alleles code for variant forms of the same enzyme. This gene has 3 different alleles. Any gene can have many alleles because many different variants of the same DNA sequence are possible. Note that dominance relationships depend on structure and function of gene product, not gene itself. The obvious thing is to do the crosses and look at the ratios of phenotypes.
Roman numerals refer to generations.
Then you try to assign each individual in the pedigree a genotype consistent with this assumption. If it works, that means condition could be autosomal dominant. If it doesn't work, you've ruled out autosomal dominant. You try to narrow down the possibilities as much as possible. You test for what is possible or not, not whether proportions from each individual cross fit the expected.