Dogs are not like humans where they miss a season (or menstrual cycle in humans) when they are pregnant. They get pregnant when they are in heat. The heat cycle and menstrual cycle are almost the opposite of each other.
All dogs do vary somewhat. I have a girl that come into heat about every 12 months or so, give or take a month. Some come in every 6 months, some every 8-9 months. Since your dog is only 3 years old she probably hasn't had very many heat cycles. It is unlikely that she will always come into heat on a certain month every year of her life. Lots of things influence when they come in, including stress levels, weather (in some areas of the world), diet, etc.
If you are not completely clear on how the heat cycles of a dog work (ie. they can only get pregnant while in heat and do not skip a heat cycle because they are pregnant) I would highly recommend reading up on it and getting your girl spayed. Spaying will lower the danger of developing mammary cancer and eliminate the danger of a pyometra.