You can get do short HIIT training with less time. But if it takes you 3 hours to get a good workout in then you might be going to easy
HIIT may not always work for some, and those who are wanting to try it should already be at a reasonably high fitness level. Hiit drains a lot of energy out of you and you may feel nauseated or lifeless after the workout. You should exercise until you are satisfied with your workout and have a rest.
Any workout could be draining. You gotta push yourself!
45mins a day at a moderate to high intensity is enough.
Everyone is different. Talk to your dr to see what your body can handle. If you don't want to talk to your dr, you probably already know you are doing too much. Be smart and be healthy!!