Cars have changed so much, and are different between year/make/model/options that an answer would be very model specific. In an F-150 I have no experience, but most newer transmissions you can select a gear and its pretty close to a manual in that it stays in it unless its going to cause damage. GM's 6 speed automatics have to be going fast enough to be shifted into high gear, but some of them can be started in 4th. A lot of older style transmission that have stayed in production can't be started off that high, and will just act like a 3 speed auto if placed in 3rd.