You don't have to include the third digit if you know the gender(s) of the animal(s).

Of course you CAN include it, if you wish, but if the third digit is zero, it's not really necessary, IMO.
You can use my signature as a reference, if you would like. I'll translate it in case that helps...
4.5 cornsnakes = I have 9 corn snakes (4 male, 5 female)
1.0 MBK = 1 Mexican Black Kingsnake (male)
The zoo = 1 dog (male), 2 cats (both male), 4 sugar gliders (2 male, 2 female), 1 husband (male), one human child (male)
Hope that helps clear things up. It was a little baffling to me when I first joined, too. I swear there used to be a Sticky about it, but I can't seem to find it at the moment.