I own 1RX.us
I've tried to have this evaluated so many times, but no-one has bothered to lend an opinion on here. So I'm guessing its not worth much... not enough to warrant an appraisal around here anyway.
Rx can refer to medical prescription. It's a Latin short form for "recipe". Doctors usually write prescriptions using this. So its like a doctors gives a "recipe" for medication to be issued to you by the pharmacist.
Personally, I have no plans to drop my 1Rx. I think it could mean "number 1 Rx" or something along those lines could work. In your case 3RX could translate to "Free Rx" - ie. free medical prescriptions?!
Since I don't know the value of my own domain, I don't think it will be appropriate to give you a price for yours. But, I'd say its worth more than regfee. Not a great deal more... but just more.
Let me know if you make a sale or get an appraisal. Good luck with it.