I have a cough and I found a 3yr expired bottle of promethazine in my medicine cabinet, how safe and/or effective would it be if I took some?

Worthless. To begin with it is too far out of date to know how stable the chemicals are. Secondly. this is an antihistamine with little if any benefit for a cough. About 30% of perceived benefit to some products is a placebo effect. Try a little honey with lemon juice, old rx but works for some.