cough and cold for 3 weeks and probable without fever as you have not mentioned is most likely allergic and your doctor has given you some antiallergics which have not helped you.see if ther is a family history of any allergy like skin allergy or respiratory allergies like recurrent sneezing or coughs then your childs cough needs to be evaluated .check the childs nose and ears as an ear infection can also cause cough,is the cough more in day or night ,does the cough increase early morning or late night,is the child feeding well or not all these need to be looked into.
Next Steps
meanwhile try using a nebulizer ,with budesonide 0.5 miligrams and livolin 0.3 miligrams 2 to 3 times a day and see if it helps or not.
Health Tips
viral infections do get over by 7 to 10 days ,any cough which lasts for more than 2 weeks needs evaluation.