That's a little aggressive, especially for a first trip in August - the hottest month of all in FL. be prepared for that, not sure where you live but a FL August is HOT and humid, even if you are used to a lot of walking - doing it in 98 degree heat with a 100% humidity index is a whole 'nother kind of h*ll (literally). And another poster is correct, there is no bus from the BC to any part of Epcot. Only way to get to Epcot is the walking path or a boat to the World Showcase entrance. Same for the Skyliner, the station is at the back of the park, in World Showcase, so near the Beach Club. You can definitely ride it, but it's going to drop you off at the same place you board, it won't take you to the BC. I would highly recommend "relaxing" your plans (for the other parks, too), it's going to stress you out when something may happen, and it will. For early entry, it's time enough to get one attraction done when at Epcot due to the distance between them. You won't get all three done in that time, unless you plan to buy
Lightning Lane access, then you might get all three done in about 2 hours, if you can time it all right. I always tell people who are visiting for their first time to have an idea what they want to see and do, then focus on what it most important. Then everything after that is a bonus. Stressing about having to be "on track" won't be fun with 3 hot exhausted kids who just want to go back to the hotel and swim - and very smart to build that in! Have a great trip, just don't be too hung up on sticking to a master plan.