Yes, that's true PROVIDED your test scores are normally distributed.
If they are, then more than 99% of your test scores are within three standard deviations of the mean.
You told us that 3-sigma = 5.4 . So less than one score out of every hundred tests you take is lower than 86.6 or higher than 97.4 .
You are remarkably consistent, so I guess Jupiter must have been in its ascendancy next door to the house of Andromeda about an hour before you were born, or some such nonsense.
