Just to repeat a test I made many years ago, I just took 3 random choice On-Semi 7805 T0220 regulators purchased in a lot of 10 from Farnell and mounted them all on the same heatsink, no heatsink thermal compound, no extra time finding an insulating washer and insulating spacing material required if I was trying to make things hard for myself and need a TO-220 PNP power transistor or when the other suggestive alternative, sharing the real estate in combo on a heatsink with an LM317, (that combo of parts takes some time more to assemble that I didn't have to spend).
Used for input an 18.5V 4 amp laptop P/S, )didn't have 12V or 15V handy), but this is even more stress on the 7805's.
Used a 220uF/25V cap to bypass the input V to the 3 musketeer 7805's, loaded them with a 2-ohm 25W iwirewound in parallel with some misc 2W resistors metal film to trim the combo to the approx 1.67 +- 5% ohm odd value,
Measured the unloaded output of each reg before assembly and the results were 5.00V, 4.99V,4.98V. (These <.3% from 5V are typical and what I always expect to get from any modern production of 7805's)
Attached a 2-in #22 wire form each 7805 reg output to a common +5V output point so as to function as a current measuring shunt to check results.
Result: 4.98V at +/- .01mV(limits of my Fluke DVM) at timed test intervals, T=1Min, T=15Min T=40Min when I decided it was enough to prove the point, beautiful 7805 5V clean output on my 20MHz scope and some of the chill somehow disappeared from my work room.