The Bb major concerto is almost sight-readable...musically it's another matter though.
The C major concerto is actually quite difficult technically...mind you the cadenza(s) are much easier than the one Beethoven wrote in later life for the Bb major concerto. I remember certain passages of the C major concerto having a 3-voice counterpoint in one hand...that can be a bit tricky. I don't recall anything similar in the Bb concerto.
In any case, if you're speaking strictly of the music's technical difficulty, I don't think that the 3rd concerto is any more difficult than the 1st...even the Emperor concerto isn't that tough from a strictly technical point of view.
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Asked if he had ever conducted any Stockhausen,Sir Thomas Beecham replied, "No, but I once trod in some."
there's some octave glissandos too in the first concerto, right?
I disagree about the 3rd not being difficult, the coda in the last movement and several spots in the first movement need lots of work - lots of recordings I've heard do the first movement too slow too. it's marked allegro con brio after all, and in that tempo, you'll certainly get stuck in the double-note passage and other places.