Hello again,

Here are the results for two regulators where one of them has 1 percent low voltage.
The nominal voltage of each is 12v. The two series resistors are 0.1 Ohms each.
The total resulting output current is 4.8 amps, the output voltage is 11.70v.
This data is I1, I2, Vout:
[3.036734693877552,1.836734693877551,11.69632653061225]

So 3 amps is drawn from one regulator, about 1.8 amps from the other.

You could hand select a resistor for the higher voltage regulator if you like.

 

That circuit is straight out of the LM138 data sheet. (edited model #)

But what's missing is the note that states the minimum 100 mA load.

As far as calculating the voltage drop across R3...Why? Are you worried about dissipation?

100 mA? Never seen any of these regulators requiring that much. 10 mA is common value even 5 mA (typical 3.5 mA) which I've checked right now in the datasheet.

EDIT/
The 100 mA seem to be required by the circuit in the OP. I understood you were talking about the regulator alone. Sorry for the eventual confusion.
EDIT/