Buy a fourth thermometer?!
Have you tried immersing it in a glass full of ice with water in it? If your thermometer will measure temperatures of 32°F/0°C, you can test it using an ice bath. An accurate thermometer will read 32°F/0°C in an ice bath at any altitude or atmospheric pressure. Fill a glass with ice, then add just enough water to cover the ice, but not so much water that the ice floats.
After a couple of minutes, insert the thermometer stem or probe into the middle of the ice bath and stir gently. Don't let the thermometer rest against the ice or you'll get a low reading.
If the thermometer goes just over 212°F/100°C, put it into a wide pot of boiling water to verify it at 212°/100°C.