The only time I've ever seen 3 tamas in one Tama-Go toy is when my character got married via Date Place (2 tamas on one screen) and later on mated (2 adults + one baby). Other than that, I don't think there is any way to get 3 characters in one Tama-Go toy.
It's only possible to raise 3 characters in one toy with the V5 Family and V5 Celebrity (aka V5.5).
~shawdy~ :furawatchi: