This is a crude method, assuming that you do not have FMS. Draw equal circles centred on A, B and C, of suitable radii eg each circle could equal half an hour or one hour or whatever was appropriate to your speed (be it all-engines or engine-out). Where they intersect your track should enable you to eyeball equi-time points. Of course, if you want to really finesse it, the radii will have to represent ground speed achievable TO either A, B or C and of course that is a problem with an off track situation. Better to use still air radii and apply common sense with the wind component.
Crude, but accurate enough, given that anytime you have an emergency, you can quickly look at where you are and then point the aeroplane at the nearest place while you deal with the problem. Much simpler with FMS, just hit the Direct to key twice (some installations) and read off which is closest. Don't execute it until you've thought about effects of wind and destination weather etc .