a) 2 amps, both input 1 with Rin=1Meg with "Y" cord - guitar "see" the two 1Meg loads in parallel, or 1Meg/2=500k. Three amps - 1Meg/3=333k.

b) 2 amps, "daisy chaining" - guitar in input 1 (Rin=1Meg), then out through input 2 to the second amp and its input 1 again (Rin=1Meg). BUT, we now have two 68k resistors in series between inputs 1&2 on the first amp, or in another words, guitar "see" 1Meg in parallel, then 2*68k=136k in series, then 1Meg in parallel. The resultant resistance is now (1000k+136k)//1000k = 531,8 kOhms. Not much of the difference, but if we connect the third amp on the same way (another 2*68k in series, and 1Meg in parallel) then resultant resistance is (531,8+136)//1000k = 400k, larger then "triple" Y-cord result of 333k. But, first amp get full signal voltage, and other two little attenuated signal through 136k/1Meg combinations.
Normally, this is correct if the inputs are wired at the same way, you can try it with ohmmeter, amps switched off, cables installed.

-All in all, you can try both solutions, but personally I don`t like either of them, as I said, our tiny voltage/high impedance source (guitar PUs) probably needs better solution, see splitter, etc, theory.