If a switch does not have ON and OFF marked on its toggle and has three terminals (not counting the ground), it is a three-way switch. A pair of three-way switches can control the same fixture by flipping the toggle on either switch.
If one or both of a pair of three-way switches fail to control the light, replace the faulty switch or switches. If the toggle is wobbly, replace the switch.
If you keep track of which wire goes where, replacing a three-way switch is not much more difficult than replacing a single-pole switch. If you forget to tag the wires, however, the proper connections can be difficult to figure out.
About 25 minutes if you remember to tag the wires; possibly longer if you forget
Screwdriver, side cutters, voltage tester, strippers, long-nose pliers
Stripping wires and joining them to terminals
Lay a towel or small drop cloth on the surface below the switch
One or two three-way switches, wire and wire nuts for pigtails, electrician's tape
If three or more switches control the same fixture, two are three-way switches and the others are four-way switches. A four-way has four terminals (plus a ground). Two wires connect to input terminals and two to output terminals. Before you remove the switch, tag each wire. Without tags, it could take a pro to sort them out.