Anytime someone HAS to have 3 beds it's recommended to book a room that guarantees that, and this issue isn't something they legally have to accommodate versus other handicap options.
I don't think they'll guarantee a daybed for a medical reason like this just like they wouldn't guarantee a Murphy bed at POR, because those rooms could be booked by 5 people parties. I think you'll be pretty safe requesting one. But worse case scenario you can get a rollaway bed if there isn't a daybed so there really isn't anything to worry about. Maybe bring the blow up mattress as a backup if possible.