There is nothing rude about a couple booking a window and aisle seat, hoping the middle seat remains empty. There is nothing rude about asking the singleton who had to book the middle seat if they might like to trade for the aisle or window seat so the couple can sit together.
What would be rude is not accepting the singleton's 'no' with grace or talking over the singleton the entire flight if the singleton is still in the middle, whether or not the couple offered to switch seats.
As a singleton, if the couple offered to switch seats, I'd happily take the aisle. I wouldn't, however, take the window. I need to get up to use the washroom fairly frequently so I would prefer to only disturb one person, rather than two. I do try to book my seat on-line as quickly as early as I can so I can get an aisle seat all the time. I haven't yet had to take a middle seat. (One airline I fly lets me book my seat when I purchase the ticket; the other lets me go on-line 24 hours before the flight and check in and book my seat.)