I've been on the pill just for 3 days, i last had my period 2 weeks ago and had unprotected sex last night, will i need emergency contraception?

Yes: If your period was 2 weeks ago, you were almost at the time of ovulation when you started the pills and may not have been able to stop it with just 3 days. To be safe, emergency contraception wold be a prudent move on your part.

Use backup x 7 days: The evidence show that the pill is effective after 7 day. So until then, you should consider emergency contraception. Se ec.Princeton.Edu.