15X is enough (though a bit on the low side). More important than magnification is the quality of the scope.
On some days the mirage is really thick, visible and easily apparent. Any scope or set of binoculars will pick it up (in fact on some days I can even see the mirage when looking through my iron sights).
On some days the mirage is quite subtle, and almost not visible at all. Sometimes on days like this a really high quality rifle scope, or even better a really high spotting scope, can pick out enough details so you can see the mirage and figure what the wind is doing.
Buy as decent a quality rifle scope as you can afford (fitting it in with your other spending priorities). No need to get a $2000+ wonder (though don't let me stop you if that's what you'd like), but it is worth getting at least a "medium quality" rifle scope.
For long range shooting it is nice to have a bit more magnification (like 20X, 25X or even a bit more). If you really do need to be able to go down to 3X, then a 3-15X is probably the right magnification range to get. But if you're able to handle a little bit more magnification at the lower end, it would be worth going to a 6-20X or 6.5-25X etc., if that will work with the kind of shooting you'll be doing (for slow fire prone, 'more magnification is better' most of the time)
If you can *also* get a spotting scope, to set up and use at the same time as you are shooting with your rifle scope, that's an even better setup for seeing mirage and reading wind (for some of the more complex wind situations with varying strengths and angles at different positions down the range). It is not essential, but it is helpful if you can.