Kid kept falling sick

Hi mommies, recently my 2.8 lo kept falling sick (fever, runny nose, block nose). Latest episode was since end of Oct until now so about 1.5 months already. 3 times high fever (between 38.5-39 degree) and after fever followed by runny nose and block nose. Until now she still has a little bit heavy breathing when sleeping (like snoring sound). She still not attending any school yet. We are a bit worried that her immune is not strong enough and once she goes to school next year she will be very easily falling sick. What should we do to build her immunity? Currently we are giving her probitoics and fish oil.