For a simple 3-band EQ, I might suggest trying lowshelf, peaking, and highshelf for your biquad filters. This way, you can eliminate all those extra gain nodes, since these filter types accept a gain value - and you won't have to split your signal. The filters can just be connected in series.
By eliminating all those splits and maintaining a single signal path, you guarantee that nothing can get out of sync.
Plus, when you connect in parallel, you're more likely to get phase weirdness. biQuad 1 is trying to remove your lows, but they're still present in biQuad 2 and biQuad 3. So your filters are sort of fighting eachother.