If you are REALLY clever you put in the 3 as the first attempt. If your answer is too big then you try with a smaller number (the 2 or the 1 for those struggling!) and if it is too small then you try with a larger number (the 4 or the 5!).
I would go as far as saying if you using trial and error then you have selected the wrong technique - just divide 6 by 2!
One poor chap I spoke to said he was asked for the square root of 441 whilst on an ILS in a sim check. He got it right too, because it is easy if you know how.
These questions are really a test of mental organisation rather than arithmetic, as many of them don't actually expect you to do the sum, but to utilise the basic relationship between operations and numbers.