If your dating someone 3 years older than you, what do u think my parents would say?

Well, it all depends on how old you are now. If you were 13 and your partner 16, it might not be a good fit. If you were 21 and him/her 24, however, it wouldn't be a problem. As long as you guys are on about the same level of maturity, it's all good.

Put simply, if you guys are teenagers, then they probably might oppose.  

If you guys are older and more of the same level mentally, then why not?

But honestly, the only way to know is to talk it out with them. 

Hope this helps! Have a great day :)