Fountek's are a bit smaller - they are like 2.5" if you draw a circle around the center of their mounting holes as they are square - for example the Memphis are a touch oversized compared to the factory E53 dash drivers but they will still fit -
Are the HK ones you are talking about the pods that have the midrange speaker and the tweeter in the same setup? If so, the Vifa 65mm drivers Jay has, along with a tweeter from parts express will work in that location - there are a few things from Parts Express in this link -
Tweeter/Midrange replacement - E46Fanatics