You have a 4-way switch. That normally works with 2 3-way switches and as many more 4 way switches as needed for the desired number of switches to control a light. The fact that you only know of one more location might mean you haven't noticed the other 3-way switch, or that things have been modified since the original installation.

Your safest bet is to get another 4-way switch and wire it up exactly like this one. At bare minimum you need a 3-way switch to have two switches controlling one light, but since that is NOT what you presently have, it might require considerable investigation to determine what wires to use on it.

You have a 4-way switch. [called 3-way outside of North America] That normally works with 2 3-way switches [called 2-way outside of North America] and as many more 4 way switches as needed for the desired number of switches to control a light. The fact that you only know of one more location might mean you haven't noticed the other 3-way switch, or that things have been modified since the original installation.

Your safest bet is to get another 4-way switch and wire it up exactly like this one. At bare minimum you need a 3-way switch to have two switches controlling one light, but since that is NOT what you presently have, it might require considerable investigation to determine what wires to use on it.