2+2=4. And 3:1 + 3:1 = 9:3:3:1
Ratios and probability are among the most commonly-referenced mathematical operation for genetics. In this module, you can combine individual 3:1 ratios and predict the offspring. The number of phenotypic classes will be 2n where n=the number of genes. If there's only one gene, you have two phenotypes - e.g. Figure 2, Chapter 3; round vs. wrinkled seeds OR yellow vs. green.
But if you combine these experiments by crossing round yellow seeds with green wrinkled ones, the results is four phenotypes (22=4). Looking at one trait each (seed shape OR seed colour) shows the underlying 3:1 ratios.
You can see in the figure to the left that there's a 3:1 ratio of round to wrinkled seeds and also a 3:1 ratio between yellow to green.
Note also that a testcross (a cross in which one parent is homozygous recessive) would show all dominant traits if the other parent is homozygous dominant, or 50% dominant traits if the other parent is heterozygous.
Ratios of 1:1, 1:0, 1:0:0:0 are possible - try out some Punnett squares of your own to investigate these.