The three registers corresponding to a single EQ band coefficient must be written consecutively, but it does not matter if they are written in a single I2C transaction or multiple.
For a single transaction, send the slave address, and then the first register address followed by the three coefficient bytes. For multiple transactions, send the slave address, then the first register address followed by the first coefficient byte. Repeat for the next two registers without addressing other registers in between.
Note: the EQ band coefficient registers can only be written when the part is in shutdown.