How do you round 0.013946 to three significant figures?
Significant figures are the first number(s) reading left to right that is non-zero.
In this case the 1 is the first significant figure, then the 3 and the 9. The 0's after the decimal point are to be left as they keep the other figures in the correct places. Looking at the digit after the 9 which is a 4 meaning that we round down.
Our answer is 0.013900 which is just 0.0139
Rules of significant figures,
1) Zeros before any number (1-9)
2) Zeros after any number (1-9)
Then, round-off the third