The Ohms law of (Vs - Vf) / I = R only works when Vs is higher than Vf.

While the datasheet for the led will tell you what If should be when Vf is 3V (look at If graph), it is not exact. As diodes vary from batch to batch, or even from diode to diode in a single batch, there are two ways to find out what the actual current is.

Since the Vf and the If of a diode are very closely paired, providing an exact level of one will provide an exact level of the other. So the first method is a Constant Current Source. Most common regulators can be put in Constant Current mode, and setting it to around 16mA with a pot should give you the Forward Voltage that matches 16mA. Adjust until the measured voltage is 3V exactly and you will know what the resistance of the pot is and conversely, the current.

The simpler method, use a ammeter/multimeter in current mode in series with the diode provided exactly 3V.

The Ohms law of \$\dfrac{Vs - Vf}{I} = R\$ only works when \$V_s\$ is higher than \$V_f\$.

While the datasheet for the led will tell you what \$I_f\$ should be when \$V_f\$ is 3V (look at If graph), it is not exact. As diodes vary from batch to batch, or even from diode to diode in a single batch, there are two ways to find out what the actual current is.

Since the \$V_f\$ and the \$I_f\$ of a diode are very closely paired, providing an exact level of one will provide an exact level of the other. So the first method is a Constant Current Source. Most common regulators can be put in Constant Current mode, and setting it to around 16mA with a pot should give you the Forward Voltage that matches 16mA. Adjust until the measured voltage is 3V exactly and you will know what the resistance of the pot is and conversely, the current.

The simpler method, use a ammeter/multimeter in current mode in series with the diode provided exactly 3V.