The call of Plain Flowerpecker is actually fairly distinctive from the other two species. From your description, I think this may be the species you heard/saw (remember that this is a desciption species, so please submit details if you think you recorded enough for the ID). I recently saw (and heard) Plain Flowerpecker for the first time, at Sham Chung and picked it up initially because of the different call, similar to recordings I had heard on line; I was lucky because the bird was feeding at eye level and allowed very good views at a short distance.
I agree that the calls of Scarlet-backed and Fire-breasted are slightly different, but are still very similar and are also fairly variable, making positive ID on call alone very difficult. I find the call of Fire-breasted to be slightly shorter, more frequent and less harsh/metallic than Scarlet-backed but I'm not sure I would describe it as more musical. I don't often encounter Fire-breasted, so others may be more familiar with the call.
Most flowerpeckers I find, in all habitats, seem to be Scarlet-backed. My impression is that Fire-breasted is fairly scarce and localised mostly in mature forest and may be declining, whereas Scarlet-backed is very common in forest/shrubland and even open country with large trees (e.g. Long Valley, Kam Tin, northern NT), and seems to be increasing.
