Most common and literal:
* J'ai deux sœurs et un frère. (This is the most straightforward and common way to say it.)
Focusing on the number of siblings:
* J'ai trois frères et sœurs. (This highlights that you have three siblings total.)
Emphasis on the different genders:
* J'ai deux sœurs et un frère. J'ai deux filles et un garçon. (This emphasizes that you have two female siblings and one male sibling.)
More informal:
* J'ai deux sœurs et un petit frère. (This is informal and emphasizes that your brother is younger.)
Other options:
* Je suis le/la [your gender] du milieu. (This means "I am the middle [your gender]", which implies you have two siblings)
The best choice depends on the context and the specific information you want to convey.