Formal:

* Three billionths

* Three times ten to the negative ninth (3 x 10⁻⁹)

Informal:

* Point zero zero zero zero zero zero zero zero three (This is a bit cumbersome, but it's technically accurate)

The best way to say it will depend on the context. In a scientific setting, the formal options would be more appropriate. In a casual conversation, the informal option might be fine.