Formal:
* Trio: Used for a group of three people or things.
* Tercet: A group of three lines of poetry.
* Triad: A group of three related things.
* Triple: Means three times the amount.
* Thrice: Means three times.
Informal:
* A few
* Several
* A couple of
* A handful of
* A bunch of (sometimes used more informally)
* Three-fold
Specific Numbers:
* Trey: Used in poker for the card with three spots.
* Ternary: Refers to something made of three parts.
* Third: Refers to the position of something in a sequence.
Creative:
* The magic number
* A trinity
* The three amigos (referencing a famous film)
The best way to say "three" depends on the context of your sentence.