Hi Stephen,
The good: you can do DSP with grouped zones. They must be RAAT zones, not Chromecast, AIrplay, Sonos, see here: Grouped zone DSP?
The bad: @dabassgoesboomboom is unfortunately right, it will only work well with a multichannel DAC. Three different DAC will have different clocks, they will drift, Roon will sort of keep them in sync but at the sample level, which is definitely not enough.
Your idea can work for a sub at let’s say below 500Hz, but even there measurements with a microphone are necessary to check time alignment.
So there’s no way but to build or buy a true multi-channel DAC. The low cost option is miniDSP DAC8, but I am not recommending it for high end hifi. Other options include Exasound, Okto Research.
For filters you can start with Roon Procedural EQ, but only one type of filters available (at various slopes though). A microphone is absolutely mandatory to fine tune frequency and time response of all speakers.
For filter designs and measurement, REW and Rephase are a good free alternative to Audiolense and Acourate. Roon convolution engine is very good and cope with various filter formats and configuration files.
My 2x4 active system has been developed with REW and Rephase, UMIK1 microphone. Convolution filters are implemented in Roon with a .CFG configuration file (stereo --> 8 way). DAC is Exasound E28. Very stable and works like a charm.