I have taken only one dose of dayquil which contains phenylephrine almost 30ml during my first trimester. i was 3 weeks and 3 days (fetal age) not gestational. my gestational age at that point would be 5 weeks and 3 days. have i done harm to my baby?

I wouldn't be concerned about this minimal exposure at that fetal age or any other stage of pregnancy. It is good to avoid meds when you can but a mother that can remain productive and healthy is also important. This is listed as a category c drug which just means they don't have enough data to say it's perfectly safe but existing data are not worrisome.