It depends entirely on how much much you want the third not to be the same gender, and statistical likely outcome will map inversely to said desire at nearly 1:1.
- If you already have two boys, buy 3 years worth of boys clothing in advance. If it's another boy, you've saved a lot of trouble. If it's a girl, no one will care – women are pretty universally allowed to wear whatever they like. It's also less likely anyone will be confused and ask – they'll see the signs of extreme triandritis etched into your face.
- If you already have two girls, it's not going to matter – they'll constantly dress up their smaller sibling like a little princess regardless of your intentions to nurture or subvert traditional gender norms.