My daughter is 3 months old and is exclusively formula fed. When do I need to start using water with fluoride or fluoride supplements?

Fluoride. If your water is fluoridated no need to start by adding to water. ADA recommends no Fluoride treatment for children this young. Please consult your dentist however I would not start until all teeth are erupted and please remember to brush you child's teeth and make it fun by chasing the sugar bugs.
Flouride supplements. there are recommendations by the American Academy of Pediatrics and the American Dental Association. They essentially say this, children under 6 months do not need supplementation. Children who live in areas with fluoridated community water do not need supplementation.