SPI Daisy-chain with paralleled DIN
Do I need to turn CS low, then write 9 byte to SPI DIN, then
double-trigger LDAC? Or to double-toggle LDAC after each 3-byte
transmission?


You activate LDAC after you have written the serial data into all three DACs and, you should ideally double check that what comes back on MISO is the data you wrote to each DAC. That gives you some confidence about things. You can activate LDAC straight after DAC3 data is seen on MISO.