Do I need an expansion joint for the 3" pipe that's going under the slab of my storage garage which is 72 feet long (pipe). This is the sanitary drain pipe under the slab in a non heated storage space!

Not unless your local authority requires it.

OTOH, most inspectors do require that any vertical penetrations through concrete be wrapped with visqueen or sill-seal.

The pipe stays under the slab and is the main sanitary pipe for a one floor cold storage space with three toilets with a total pipe length under the slab of 72 feet! There will be 2 vents, each by the toilets and the reason why there are only 2 is because there are 2 toilets which are back to back...

Since the pipe is under the slab and in the ground, it doesn't get as hot or cold as above the ground. The pipe will stay horizontal all the way...

With all this, are expansion joints needed ?:o