I believe there is too much variation between individuals for these to be reliable.
A method that is not too bad is to measure individual results at 5 or 6 RM as well as 3RM, figure the pounds added per rep decreased, and figure the 1RM accordingly.
E.g. if 300 lb at 5RM and 320 lb at 3RM, that’s 10 lb per rep and thus figure 340 as a crude 1RM.
Sometimes actual will be better because of a bigger jump between 1 RM and 2RM than between later reps, or less because of neurological factors being the limiting factor, e.g. Golgi tendon organ inhibition. In some cases a person’s 1RM may not be substantially higher than their 3RM – basically they shut down with anything much past the 3RM – but their potential for 1RM is.