HI I have gone through your query, as you told that you have three episodes of sore throat
within a month with increasing severity and at every incidence you took amoxicillin
initially response of antibiotic was good but in subsequent episodes this medicine became helpless.
By your this story it is evident that you have taken antibiotic amoxicillin in an inadequate dose therefore your throat infection
was not cured and results as recurrence of throat infection.In subsequent episodes of throat infection,amoxicillin became helpless it indicates that infection had now developed resistance to amoxicillin.
I would like to advice you for these basic investigations-
(1) Culture and sensitivity of throat swab.
(2) X-Ray paranasal sinuses
(3) X-Ray chest P.A. View.
A thorough clinical examination and co-relation of inferences of investigations one may reach to correct diagnosis and can plan for treatment.
I hope now you are satisfy about your query.