Are you expecting to have I2C devices with conflicting addresses?
The DS3231 and SH1106 can share a common SDA and a common SCL, so just wire those together, and likely wire the SCL straight to the Arduino, with the SDA through the multiplexer. No need to use different channels on the 74HC4067 for those since they do not conflict, and make sure there are pullup resistors on both sides of the multiplexer (the DS3231 and SH1106 commonly have pullup resistors on their circuit boards, you may need to supply the pullup resistor on the Arduino side). Until you have an I2C device with a conflicting address you never need to switch channels on the multiplexer and it is basically a useless component.
The 74HC4067 is a single-pole analog multiplexer, there are chips made specifically for multiplexing I2C.