The "Three Day Rule" was created for people dealing with door to door salespeople and was later added to cover Time Share presentations. The principle is to allow consumers to get away from a pressure sales pitch in a place where they do not normally do business and come to their senses.
The standard Real Estate Contracts we use in Washington do not have a built in cancelation period. It's assumed that if you listed your home for sale with an agent, entertained showings, received an offer and signed it, that you meant to do that.
Now, I haven't seen your contract so I can't tell you what yours actually says, but I'm doubtful yours contains such a clause.
Your options at this point would be to ask to be released, if you have a sudden change of employment or health which complicates your situation you may find a compassionate buyer. You can refuse any changes due to inspection or extensions and use the contract to your advantage where possible, making the sale unattractive to the buyer. You can consult an attorney (which you should do regardless) to explore all of your options.
If the reason you want to cancel is to accept another offer for more money, be careful. Back up offers aren't always as secure as they seem. You could end up without any offers if you get too cute.