Had a cold for 3 weeks, did virtual visit and doctor has prescribed doxycycline 100mg for 7 days. was on amox-clav two weeks ago for ear infection before sinus issues really got bad. is doxycycline the best treatment? read that it is not?

Only bacterial infections respond to any antibiotic; viruses do not. Even with symptoms for 3 weeks, it could still be a viral infection, or allergy -- in which case neither doxycycline nor any other antibiotic will make any difference. For true upper respiratory bacterial infection (like sinus infection), doxycycline and amoxicillin-clavlulanic acid probably are equally effective.