Less sound on one ear than the other

The design of these headphones includes a microphone on the outside and on the inside of each earcup. The outside microphone detects outside noise. The inside microphone monitors what you are hearing and when it detects that the outside noise is present inside, the active noise-canceling circuits remove that noise. This happens fast enough that you perceive a significant reduction in the outside noise getting to your ears.

The airspace inside the earcup is a tuned chamber defined by the shape of the foam in the ear cushion. This is important because with the inside microphone in close proximity, but at a 90-degree angle to the speaker, the circuit must be designed to eliminate the normal feedback that occurs.

If the foam in the ear cushion has weakened/deteriorated to the point that it no longer has it's original shape, (in particular the lower half of the ear cushion), the size and shape of the air chamber inside of the earcup is directing more of the speaker sound toward the microphone than the design can compensate for causing audio feedback to occur. You are hearing that feedback.

On my QC-3 headphones, I can cause the feedback squeal in both earcups, with the left earcup being more sensitive to the shape change than the right.

It is probably time to replace the ear cushions. If so, they are available from Bose for $35/pair.

QuietComfort 3 Acoustic Noise Cancelling Headphones Bose Product Support

They are also available aftermarket at Amazon.com for $16.99/pair
Replacement Earpad ear pad Cushions For Bose QuietComfort 3 QC3 Headphones...

or $5.69/pair.
Bluecell Pair of Replacement Earpad Ear Pad Cushion for Bose Quietcomfort...

The ear cushions are held on the earcup with six snap fasteners, three on each side. To remove the ear cushion, simply pull it away from the earcup carefully so that you do not break the plastic snap tabs.

It should not be necessary to pry the ear cushion off with a sharp object, but if you do, be careful. There is also a thin gasket seal between the earcup and the ear cushion, take care not to damage it.

If you need to replace the ear cushions with new ones, they are available from Bose for $35/pair.

QuietComfort 3 Acoustic Noise Cancelling Headphones Bose Product Support

They are also available aftermarket at Amazon.com for $16.99/pair
Replacement Earpad ear pad Cushions For Bose QuietComfort 3 QC3 Headphones...

or $5.69/pair.
Bluecell Pair of Replacement Earpad Ear Pad Cushion for Bose Quietcomfort...

Call Bose 1-800-999-2673 they sell replacment ear cushions for the QC3 I paid $38 for two.

Throw them away and buy some Sennhiesers you'll get 5 times the quality at 1/4 the price. Or if you insist on spending big bucks go with Fostex or Grado. 90% of the price of Bose products goes to advertising, if one of the phones isn't working it means there is a short or open circuit in one of the cables. The cables are purposly made so that you can't repair them. If you bought a new set of cables and soldered them it they might work but I would venture a guess that a good pair of Sennhiesers would set you back less than the cables from Bose.

Change the battery or batteries. If that doesn't work they have to go back to Bose.

How do replace batteries

sounds as if you have a short in the left ear. solution would be to send in to manufacture. Or return to the store.

Try phoning Sound Idea Audio and Video Store at (817)346-4500 for advice.

They will give you a new pair for $100, even if it has been longer than a year...

http://riajournal.com/ria/index.php/2009/04/25/bose-gets-5-stars-in-my-book-for-their-qc3-headphones/

same thing has happend to me, sometimes when im playing around with it i get about 2 hours use before it turns off again. do you recon its worth sending it off to bose for repair?