The LM317T needs a potential divider to set the output voltage as you have determined. There are two resistors in this divider, R1 and R2.
The LM317T controls the passes current to get 1.25V between the output and the sense terminal. The easiest way of detrmining the resistors is to decide on a current through the divider. Let us say 1mA, then Rupper will need to be 1.25k.
The rest of the output voltage is across Rlower, in this case, 3V - 1.25V = 1.75V so you will need a resistor of 1.75k to pass 1mA.
Check this with your formula. If the voltage is not critical, go for the nearest preferred values i.e. 1.2k and 1.8k.