All measurement tools are accurate until you have more than one.
My advice (that I seem to have to repeat a lot to friends) is to pick the one closest to accurate (check via ice bath and boiling water) and hide the other two. Having more than one thermometer will serve to do nothing but mind-eff yourself.
The least expensive way to get yourself a new thermometer that you could trust (IMHO) is to hop on eBay and pickup up a PT100 RTD from the China-land (if this makes you feel dirty, go to your local micro and support local business afterwards) then pick up something that can read the PT100 and display a calibrated value (like a PID) and slam it all in a project box or small electrical enclosure. This little bit of work would result in an accurate thermometer for a fraction of the cost of a lab thermo.
In my experience, stay away from the two wire (thermocouple) sensors like you find on walmart style kitchen thermometers or the kind that will plug into some multimeters. These will tend to shift their reading with time, but it occurs slowly enough that you generally don't catch it until you are mashing 5*F off.
Yatta yatta yatta, you can also just hunt down industrial/lab thermometers on eBay, but they tend to go for a premium unless you grab something that has been through hell and back.