3 tenants didn't pay rent, 2 bailed, one stayed. Do I need to serve all three with Forcible Detainers, or just the squatter? State of Iowa?

You need to serve all three because none of the three have turned over possession of the property back to you. But since the two that bailed likely didn't provide forwarding addresses, they all will be served at the same place (the rental).

Posted the 3 day notice to pay or quit 4 days ago and have pictures of it posted to the door. Tried multiple times to contact all of them but no dice. The dude that is still there is either never home or hides in the basement when I try to knock or ring the door bell. But the dogs are still in there.

Just the squatter. You can file small claims action against the other two for back rent.

File in the names of ALL people on the lease no matter if they have already skipped out or not. Also list "et al" that means and all others & will cover any unauthorized tenants they have left in the unit.

Are all three jointly and severally liable for rent? I would serve all three. They didn't pay, and they didn't deliver the premises back to you vacant, either.

Do all 3 to be safe. One may come back in the mean time.

Ask a realestate lawyer in Iowa.

In most states posting it on the door is legal.