3x8GB RAM memory with different clock frequency?

I bought 2x8GB DDR4 (2666 MHz) RAM memory.

1x8GB DDR4 (1200 MHz) is installed in the laptop.

Should I block all three and use the one with 1200MHz in the single channel or rather leave it out?

I would run the two faster ones on dual channel. You hardly need more anyway.

You will hardly be able to install 3, since normally there are only 2 slots. There are devices with 4 places, but they are rare and expensive.

Have a device with 4 slots otherwise I would not ask.