Ah...
I had this problem but it was because my wiper arm was not original (it fit on top of the shaft nut extending it out too far) - a more modern generic swap from the Previous Owner.
Walck's shows blocks for the 3A/3B making me think they're the same dimensions. I don't know if that is accurate.
The measurement provided on the 3B Page are 2.25" tall. Check to see how tall yours are for comparison.
Going back to Caprio's 3A:

Those wiper arms look the same as the 3B and they should slip down onto the shaft before the acorn nut is attached - holding it closer to the w/s frame.