Okay, I'd missed that part. Three hours is not an unreasonable length of time to be seated. Many international flights are much longer, and it's probably a good idea to split any flight of four hours or more in duration into two sitting stretches with a walking-around bit in-between. Probably the easiest way to accomplish this latter bit is simply to get up and walk to the lavatory whether or not one needs to actually use the facilities. Since there's nothing unusual about using the lavatory in flight, such action should demand no particular attention.