In a magnitude scale, the only meaningful value is the "distance" from 0, so negative magnitude values do not exist. In this case, both +3 and -3 have the same magnitude since both their distances from 0 are 3. Similarly -5 has a larger magnitude (5) than +2 (2).

If you have seen negative values on a magnitude scale, it is probably one using logarithmic magnitudes of ratios (e.g. in measuring loudness or brightness). In these cases the measurement is not being made on an absolute scale, but rather with respect to some non-zero reference. In the case of sound, decibels compare one sound to a reference , and the reference might be the quietest sound that a human can hear. A value of 0 (zero) would mean that a sound is of the same intensity as the reference. Quieter sounds would then be negative on such a scale, but they still are sounds. (Someone with better hearing might be able to detect them.) If those sounds were measured based on pressure, no sound would be at 0 (zero) pressure difference, and other sounds would be at some positive value.

Positive means larger than a reference level, and negative means smaller than a reference level. For example, for the scales used to measure the strength of earthquakes, a magnitude 1 quake has ten times as much movement as a magnitude 0 quake, which in turn has ten times as much movement as a magnitude -1 state.

If you are talking about the Richter magnitude scale those numbers are generally positive integers 1-10 which qualitatively describe the severity of an earthquake. If you are talking about the magnitude of a number - well that is the distance of the number from 0. You may have also heard this called the “absolute value” of a number. It is useful because sometimes we only care about how far a number is from 0, and not if it is positive or negative. Or we may only want positive numbers - and a good way to get rid of any negatives is to take their absolute value (or magnitude) instead.