Formal:
* J'ai trois frères. (This is the most common and direct translation)
Informal:
* J'ai trois grands frères. (This literally means "I have three older brothers", but is often used even if they're not all older)
More specific:
* J'ai trois frères aînés. (This explicitly means "I have three older brothers")
* J'ai trois frères cadets. (This explicitly means "I have three younger brothers")
You can also use the phrase "Je suis le [number] de [number] frères." This means "I am the [number] out of [number] brothers". For example, if you are the youngest of three brothers, you could say "Je suis le 3ème de 3 frères."
Remember to choose the phrase that best suits the context and your relationship with the person you are talking to.