Usually the warranty is for the original purchaser only.
True, but Sears is usually very good about honoring them.
To OP: if you look on top of the battery, there should be a sticker with the date of purchase noted somehow (often by scratching out the month/year). If that's within 3 years, they might honor the warranty.
But 427 CCA on a 3 year old battery; you've got to expect some degradation over time and if you bring that up (or they test it) it might not work out.
Good luck with it.