I resigned from my job, and have documentation that it was accepted, giving three weeks notice. My boss was very upset at me for resigning. As long as I continue to do my work, can I be fired without legitimate cause before that time? If I can, then what is the point of writing and making sure a resignation letter was accepted? Does this fall into the realm of retaliation against an employee?

Yes it would be legal to let you go earlier than you planned. No it isn't illegal retaliation.