I think I have strep throat, but can't get to the doctor for 3 more days because I'm out of town. Is it safe to wait the 3 days?

No. If you truly have strep throat the earlier you get treated the lesser the chance of complications. However strep throat in adults is rare. If you have runny nose and / or productive cough, that ain't strep.
Yes and no. Most sore throats are viral, but with strep you have a few days to have it confirmed before the complications known with the disease would be set. For most, confirmation with a strep test & starting treatment within 10 days will shut down the creation of antibodies that lead to rheumatic f ever & heart complications. Kidney issues with some strains may not be preventable even with early rx.