Lights operated by 3 switches must have two 3 way switches and one 4 way switch (always-no exceptions). I can only make an educated guess that one of the upstairs switches is a 4 way but not 100% sure.
I would highly recommend that you just replace the faulty switch right now. The 3ways and 4ways form a complex circuit and there is only one correct way to wire it. If you don't get the wires on correctly, there are a few hundred ways to hook them up and all but one won't work!
Turn off power, remove the faulty switch leaving the wires attached. If it has 3 screws (or 3 wires attached) it is a 3 way switch. If it has 4 wires it is a 4 way switch. Make a good sketch of which wire is hooked to which screw on the old switch. The sketch may help if the new switch is oriented differently. Label the wires if they are same color.
The switch (3 way) has one screw (usually brighter than other screws) that is called a common. The always hot wire (one end) or the hot going to the lights (other end) hooks to the common. The other two wires are called travelers and just go from switch to switch.
On the four way, two wires travel to one 3 way, two wires to the other 3 way.
If the bad switch is 3 way and you replace it successfully (to be sure you have to turn on at one switch, off at the next, on at the third, repeating a few times throughout. If it works most of the time but not all, you prob. Have crossed travelers.
Once debugged, you can prob replace other 3 way using the same process.
I have gone to see friends sitting in the stairway with their head in their hands because they bit off more than they could chew. Sticking to one at a time increases the odds of success.
If you are unsure which is common, a simple continuity tester will be handy. And if you have to test to find common, if you have the sketch you can test the old switch and see which wire was on old common screw, and put the same wire on new switch common (this applies only to 3 ways).
Here is a diagram:
http://ezdiyelectricity.com/images/wiringdiagrams/switches/4-way-switch-wiring-diagram.jpg
I would highly recommend that you just replace the faulty switch right now. The 3ways and 4ways form a complex circuit and there is only one correct way to wire it. If you don't get the wires on correctly, there are a few hundred ways to hook them up and all but one won't work!
Turn off power, remove the faulty switch leaving the wires attached. If it has 3 screws (or 3 wires attached) it is a 3 way switch. If it has 4 wires it is a 4 way switch. Make a good sketch of which wire is hooked to which screw on the old switch. The sketch may help if the new switch is oriented differently. Label the wires if they are same color.
The switch (3 way) has one screw (usually brighter than other screws) that is called a common. The always hot wire (one end) or the hot going to the lights (other end) hooks to the common. The other two wires are called travelers and just go from switch to switch.
On the four way, two wires travel to one 3 way, two wires to the other 3 way.
If the bad switch is 3 way and you replace it successfully (to be sure you have to turn on at one switch, off at the next, on at the third, repeating a few times throughout. If it works most of the time but not all, you prob. Have crossed travelers.
Once debugged, you can prob replace other 3 way using the same process.
I have gone to see friends sitting in the stairway with their head in their hands because they bit off more than they could chew. Sticking to one at a time increases the odds of success.
If you are unsure which is common, a simple continuity tester will be handy. And if you have to test to find common, if you have the sketch you can test the old switch and see which wire was on old common screw, and put the same wire on new switch common (this applies only to 3 ways).
Here is a diagram:
http://ezdiyelectricity.com/images/wiringdiagrams/switches/4-way-switch-wiring-diagram.jpg
