Each INA3221 has three channels and can be set to 4 different I2C addresses. That means that 12 channels are possible without extra I2C multiplexer.
In the datasheet in "Table 1" is how to set a I2C address.
For the I2C bus, you can connect all the SDA and connect all the SCL.
I think the addresses are 0x40, 0x41, 0x42, 0x43. You can run the I2C Scanner to check the addresses.
The datasheet says explicitly that pin A0 is sampled on every bus communication. That means that more than four INA3221 chips can be used on the I2C bus if a seperate digital output pin is connected to the A0 of each INA3221. When A0 is high the address 0x41, and a single INA3221 can be set to 0x40 via A0. Then each sensor can be used at 0x40 one by one.