You can hire a professional carpet cleaner to see if the stains come out, and deduct the cost of the carpet cleaner from the deposit. You need to give an itemized list of any charges to the renters that moved out within 14 days of move out, along with any portion of the deposit not applied to damages. If the carpet cleaners cannot remove the stains, you may be entitled to charge the renters for a portion of the replacement carpet cost based on damages above normal wear and tear.
Mr. McDowell accurately states the law. A few additional practical considerations . . . (1) I have some serious question whether a judge will hold that the damage is more than wear and tear given that they were three-year tenants. This will be particularly true if you do not have a move-in document showing that the condition of the carpet at move in was good. (2) The 14-day notice is not required unless the tenant requests it. (The request need not be written.) But I would suggest you comply as if the tenant gave notice no matter what because the penalty for not complying is twice the amount wrongfully withheld, and Justice Court judges can be unpredictable.