Lots of snuggles.

It seems like you already have a good handle on the rest already, even if 3 URTIs in 5 months is no fun for anyone.

It might be worth giving your house a thorough clean to see if that helps. Maybe there's a source of irritants (molds, pollens, that kind of thing) that is only really problematic for someone with a weakened immune system, and therefore more prone to secondary infections.