What values of components to use in this 30V-2V, 3A(max) current limiting circuit?
How it works:


R1 biases on Q1 and the collector current starts to increase.
When the voltage across Rsense rises to about 0.6 V Q2 begins to turn on and steal the bias from Q1.
The output stabilises with 0.6 V across Rsense.


For 3 A max current limiting \$ R_{sense} = \frac {V}{I} = \frac {0.6}{3} = 0.2 \ \Omega \$.


  Is R sense the pot?


No. This is a fixed-current constant current source. Pots don't come in low values like that. 100 Ω would be a low value in most potentiometer series.