Need help adding fan to existing 3-way switch setup
If the fan is controlled by a 3-way switch, you'll have to replace either of the switches with another 3-way switch.  Unless you eliminate one of the 3-way switches, you can't install a single pole switch.

If you want to install a fan speed control switch in this scenario, you'll have to find one that can be wired as a 3-way switch (not sure if it exists).