The panel has wiring blocks that the HV primary conractor wires to and can test to.
The loads into that block will be on the RCD, so only 3 needed, or based on load calcs fewer still as potentially all 3 dimmer packs could be on one breaker. Here in the US my lighting panel loads are on a single breaker for the panel.
You are overthinking the RCD and dimmer pack. You would not be asking if each light switch or dimmer in a traditional wiring structure needed their own RCD, centralizing doesn't change the current flow. Just the location of the parts.
The panel has a HV side (250) on the left and LV on the right to meet with electrical code.