Not cleaning an internal for 3 months makes me worry
I never had internal filters as they were a PITA because i had the sponges getting to a point where they are officially classed as 'gross' aka brown and slimy (usually plant matter than waste) but I think most internals could do with a quick sponge clean every other week at least.
This is why external filters are so much better! Varies from tank to tank, with my TetraTecs i cleaned them once a month, fluvals i had to clean once a week/every other week, the Aqua Manta every other month or every 3rd month.
Eheim won it though, had the biggest Eheim classic (aimed at a 600L tank on a 260L tank with 10 x 1" E. Canarensis cichlids, 6 little bristlenost and a few odd fish) and I took out the carbon and ran it with the tightly compacted 5 sponges alone and i opened the filter once a year to clean it, it kept the tank going really well, flow never dropped or anything! However, i still hate eheim filters, its motor was noisy/rattled, the green of the filter/piperwork is just ugly and god was it a nightmare to clean! Tiny flimsy metal clips holding the top on and to disconect it, i hat to put my thumb over the pipe to stop the water emptying out the base of the canister! Never again!
Also, if you havent cleaned it for 3 months, does that mean you havent changed the carbon/wool filters?