I don't believe there is a stock, or even stock appearing, off plus 3 run position switch.
Use the stock dash switch and pick two speeds you like, or you could let the first position of the dash switch feed a hidden second SPDT switch to let that dash switch position control the two lower speeds by flipping the hidden switch while the second position of the dash switch turns on the high speed. That's how i did it.
Removing the dash was a chore but made the blower swap so much easier and you'll find a bunch of details to tidy up back there while you've got it opened up.
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