Theoretically, 3.5cms is desired. But clinically, many doctors take a cut off of 2.5to 3.
With a value of 3.1, it's borderline. You may avoid unnessary travel. And just get a second opinion.
Your anomaly scan at 18-20weeks will give further picture whether is it maintained at that or changing.
Next Steps
may take pregnancy support medicine(susten) if your doctor advises.