On my Buescher (also curved and keyed only to Eb3), E3 is incredibly easy to get. All you have to do is add the G key to the Eb3 fingering. For F3 and above, I start out with left hand 1 and 3 (B and G), octave, right hand 1 (F) and side Bb. To get higher, release right hand 1. For the next note, add side C. To go even higher from there, try left hand 2 and 3 with right hand 1, 2 and 3. Then go left hand 1 and 3 and right hand 1 and 3.