There's transportation from every park to the other parks, but it's not always a direct bus. At MK, you have to ride the monorail to the TTC and transfer to the Epcot monorail to get to Epcot. In fact, you may have to go to the TTC to get to the other parks, but I'm not sure. I just don't recall seeing park bus stops at MK. The transportation between MGM and Epcot is by boat, and it goes to the International Gateway instead of the main entrance at Epcot.