I did the same 3 places.
Apart from the imperial tombs, there really isn't much to see in Hue.
There is the Imperial Palace, but when I went 2 years ago, 90% of it was still under construction or simply non-existent. I was expecting something like the Forbidden City in Beijing, but it isn't at all. Most of it was just ruins or simply empty land where the buildings used to stand.
The tombs were OK, but not really must-sees. Historically and culturally, Vietnam is like a mini version of China, due to 1000 years of Chinese influence. But don't mention this to the Vietnamese, because they probably don't like being reminded of this subjugation by China.
If the tombs and palace ruins are all you are seeing, you can do it in 1 full day (half day each). In fact there are a lot of package tours just for that.
Danang is probably more interesting, with the beaches, Marble Mountains, the Dragon Bridge (the giant dragon breathes fire and water at 9pm on weekends), Han river promenade, Ling Unh Pagoda. 2 whole days should be good.
Hoi An is very pleasant and relaxing. Lots of good, cheap food and alcohol. Rent a bicycle and ride around.
You can also join the free Hoi An bicycle tours to the countryside (
http://www.hoianfreetour.com/). Contact them to reserve a place. Just rent a bicycle and join in. This free tour is conducted by University students who are learning English. In exchange for the free tour, all they ask is that you converse with them in English so that they can get some English practice. Win-win I say. I did the tour and it was great. You get to see the countryside, learn abit about their history, meet other travellers and have a nice chat with really friendly Vietnamese undergrads. There's also a food tour (payable) conducted by the same company, but I didn't try this.
Are your days full days, or do they include traveling? Your hotel can arrange shuttle bus to bring you from Danang to Hoi An. Between Danang and Hue you can also take a shuttle or train. I took the train, but didn't think it was worth the hassle and time.