Can I cancel my contract with a realtor within 3 days of signing, without consecquence?

You should tell the realtor you had a change of heart.  They may ask if you have a potential buyer already. If you do, they'll probably want to be paid something if the sale goes through.  If you do not, they might agree to let you out of the deal if they've not spent any money or taken any action to market the property.  But if they won't agree to void the contract, you are probably stuck with the contract.