No I don't think your mad, what I think we forget as adults is to make the journey an adventure as well. He'll probably sleep most of it. As long as the coach has a loo you'll be fine. I did a 1 hr coach journey with my 3 year old, took snacks, toys etc.... He fell asleep on my lap. If also look at trains, if you book early you get good deals. That way if you reserve seats you know you'll have a bit more space.
DS is 3.9. He's used to long car journeys (2.5 - 5 hours) with only 1 or 2 loo stops, and tends to sleep a lot in the car.
We're in the East Midlands and my work are running a coach trip in the summer to go to London for the day to see the museums. I'd love to take him, so he can go to the Natural History museum, see the buses, tube trains etc, but I'm not sure about taking him on a coach. He'd have less space than in a car, where he can have stuff around him on the back seat, and can't get up and walk like he could on a train.
He'd probably love the chance to go on a coach, but I'm not sure my nerves would stand it. Is it an utterly mad idea to take a 3 year old on a coach trip like this?