There is absolutely no way the orbiter would be jettisoned from the 747, that is completely senseless. If the 747 has enough power to takeoff w/orbiter, it should easily maintain altitude on 2 engines, much less 3, just like any plane can fly perfectly well on 50% of takeoff power once aloft. Single engine loss of takeoff is also not likely to cause an issue, as it only operates from the longest runways; loss of two would probably end badly, though. I'm sorry but these things should be common sense for pilots.
Also, to answer some earlier questions, the max L/D of the orbiter is about 2, and the outer glideslope is 18°-20° depending on weight; typically flown at 300 KEAS and a transition to a 3° inner GS is started around 3000 ft AGL with touchdown around 190-210 KEAS. Empty orbiter is about 175,000 lbs, which I think is much less than useful load on a 747; of course it adds a ton of drag so the range is low probably because they can't carry much fuel and the engines are not efficient at low altitude.
Also, someone asked wether the orbiter could take off under its own power; well the original design had it with air-breathing engines that would be used after entry and for self-ferry flights, however the added weight of engines and jet fuel ruled that out because it really ate into the payload. Also it originally was supposed to have been and even poorer glider (just a lifting body) but the DOD required that it have 1000 nmi crossrange (gliding) capability from orbit so that meant it had to have big, heavy wings, which made everything bigger and less efficient. Then the DOD only used it a couple times and NASA was stuck with it the way it was.
