I'm afraid that what you've got is just a 3 speed transmission -- there ain't no 4th gear for it to do a 3rd shift into. Even the manual transmission cars of the early E21 series turn a lot of revs at highway speeds because the manual has no overdrive 5th gear either. This was the era when the Germans were still engineering gear ratios so that peak power in top gear corresponds to top speed, and so cruising is a bit noisier than when they started to add an overdrive top gear.