Many folks believe they have a 3-day right to cancel any and all purchases, but the Texas Attorney General explains that this is not the case:
Many consumers are under the impression that they have a 3-day right to cancel any and all consumer purchases. However, this is not true. The right to cancel law applies only to very specific situations.
The 3-day right to cancel law applies to sales made at facilities other than the seller's place of business. Such locations may be the consumer's residence and places rented on a temporary or short-term basis, such as hotel rooms or convention centers.
Generally speaking, once you sign a contract, you are bound by the terms it contains. There may be exceptions (e.g., a contract was signed under duress, there was deception or fraudulent activity), but only an attorney can provide a legal opinion and inform you of your options. It is also possible that a contract contains a cancellation clause or a "cooling-off period," so you should review the terms of your purchase agreement to see if it describes a return policy. It may also be possible to negotiate with the seller and come to a mutual agreement.