Understanding current readings from a multimeter
On the 10A range, your multimeter probably won't have great accuracy with low current, but your measurement sounds reasonable. 0.17 to 0.21 A, or 170 to 210 mA.

Presumably your air pump is a small DC motor, which will have different current measurements at startup, and during running conditions with different loads.

To calculate resistance, you use Ohm's law:

$$R = E / I$$

If the voltage (E) is 3, and the current (I) is 0.17 A, then:

$$\frac{3}{0.17} = 17.6\Omega$$

Therefore, your resistance calculation seems correct. Just keep in mind that a DC motor will measure different resistances (currents) depending on what it is doing, due to induction/reactance.