Assuming you are referring to a rifle scope, the 3x refers to the amount of magnification the scope provides. So a 3x scope would be good for hunting or target shooting at medium to long range where you need more magnification than what is provided by iron sights but don’t need the extra power of a higher-magnification scope.
A 3x scope will make your target appear three times closer than it does with the naked eye, which can be very helpful in picking out details at longer range. However, this also means that your field of view will be reduced by one third – so you will have to be more precise in your aiming.
At shorter range, a 3x scope can actually be a hindrance as it can make targets appear larger and more distorted than they really are. This can make it harder to hit your target, especially if they are moving. If you plan on doing most of your shooting at short or close range, then a red dot sight or an unmagnified reflex sight might be better options.