Replace ALL the batteries with 15 identical new batteries - don't ever replace just one, and certainly NEVER with a different make or type.
There should be no issues using 3A batteries in place of 2.6A (as long as all are identical and new), and the 3A rating may be fairly imaginary anyway.
One suggestion I would make is to measure the voltages on all the new batteries, and insure they are all pretty well at the same charge. We repair a LOT of battery packs, ordering batteries in hundreds, and often find some are more charged than others, and this leads to premature failure of the new pack. So check them all, and if they are different, then charge each one separately to make them the same.