Can plan b lose effectiveness if i finished an antibiotic 3 days ago? still on birth control pill

No: In general you do not need to take Plan B if you were already on birth control. Most antibiotics will not interact with either Plan B or birth control. If you are uncertain in the future you can always ask the pharmacist or just use a condoms during those days.

Plan B does not: interact with many antibiotics. Antibiotics which can affect Plan B: ampicillin, cotrimoxazole (in Bactrim), Rifampin, tetracycline, & clarithromycin.