This review is a warning for anyone considering purchasing the Jaybird X3 headphones, I have had two pairs of these headphones, and have had a very similar issue which might very well be a systematic hardware fault.
I had my first pair for four months before the left ear bud stopped receiving sound. When I received my second pair, straight out of the box, they had a similar problem; the left ear bud was receiving sound, but just at a significantly lower volume than the right ear bud. It was very disorientating and unpleasant. Other reviews mention similar issues with the left ear bud so it appears that this could be a fault of the model.
It pains me to give these headphones such a low rating, I really do think that these are the best headphones I've ever used in terms of audio quality, but if they can't reliably function for 4 months before breaking (or being faulty out of the box!) then they aren't fit for purpose and shouldn't be purchased without sufficient warning. I wish I had an alternative brand/model to recommend instead but I don't, I am very disappointed with the apparent lack of build quality of these as it really lets down a fantastic product. Buy these at your own risk, if you do I wish you the best of luck that your headphones function properly, and if they do, know that I'll be jealous of your functioning Jaybird X3 headphones.