Let's say your time variable is
time = 0:.1:10;
and your data is named 'data'.
First locate all time samples between 0 and 3 sec
idx = time >= 0 & time <= 3;
Then calculate std of your data for those times.
sig = std(data(idx));
This can be done all in 1 line like this
sig = std(data(time >= 0 & time <= 3));
*This assumes your 'time' and 'data' points correspond.
