I have $3000 USD to invest, is it worth it?
What sort of returns should I expect
Don't "expect" anything as it could turn out good or bad. Only gamble if you are ok with any possible outcome. If you can't afford to lose the $3000 then simply don't gamble. If you do decide, online Bitcoin casinos is a better option than a real casino. Try out Bitcoin dice sites where you can choose your win chance and try out different methods.
I think the OP wants to invest in gambling sites, not gamble on them... I personally think investing in an online casino is a way better decission than playing on one... That being said, even investing in a casino might cost you money (the casino might vanish, get hacked, somebody might win a lot, altcoin prices might drop in just-dice's case)