To some degree, these things are a matter of personal preference.
In my opinion, there is no point in even going to Epcot if you are only going to spend 3 hours, it would be a waste of the energy needed to get there. Figure you'll spend the better part of two hours getting to the park, and leaving it, only to spend 3 hours in the park. You can't even see all of Future World in 3 hours. Three hours might be enough time to tour ONE SIDE of Future World, but you aren't going to see all the details, just hit the big things and move on. You certainly won't have enough time to explore World Showcase. Three hours in the World Showcase is perhaps enough time to see one or two countries reasonably in depth. You might sneak in a third country in 3 hours if you just skim over them quickly.
If it were me, I would skip AK, and even DHS before I would consider only spending 3 hours in Epcot. On our first trip, we had two full days in Epcot, and came away feeling as though we had really missed a lot of what the park (and especially the World Showcase) has to offer. Last year, on our second trip, we spent four days in Epcot (full days, from opening to closing). We still didn't see every detail, but we came away feeling that we had seen much of what the park offers. This coming summer, we will again spend four full days in Epcot.