Generally there's no 3 day "cooling off" period, or right of "rescission" (cancellation) unless the contract at issue is a mortgage, or a home improvement contract, or a door-to-door sale, or unless the contract itself says so. Here it sounds like your contract only gave 48 hours to cancel and get a refund, which could be 48 hours more than some sales.
If the manager hasn't gotten back to you yet, I'd make an appearance at the store, find the salesperson who misinformed you about the 3 year warranty, and have a face-to-face talk with the manager. You don't have as strong a position as they do, but it'll be harder for them to say no to you if you confront them in person. Also you could remind them that word of mouth is very valuable these days, and your use of social media sites and review sites could be very helpful to them, if they're understanding and cooperative with you.
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