NECA Technical have received a number of queries regarding three wire connection of RCDs. A RCD will still function without a neutral connection, however the test button will not operate. This can be overcome by installing a resistor between the load side neutral terminal and load side phase that supplies the trip button circuit - see wiring diagram below.
There will be a wiring diagram in the installation instructions supplied with the device and these should be referenced to ensure correct connection for the specific RCD. Your supplier will be able to supply the correct resistor for the RCD to be installed.