I just found out that I'm 3 weeks pregnant and I've been drinking alcohol and taking cold medicine for a cold. Have I harmed my unborn child?

You can't go back . Congratulations on your pregnancy! it is impossible to know but the chance of significant harm is low. Whatever done is done--you can't go back. I would stop all those now and start taking prenatal vitamins, eat healthy, exercise regularly, don't smoke/do drugs, and course consult your doc/ob physician for evaluation and plan for the remainder of your pregnancy. Best of luck to you.