He may have allergies, or he may be starting an upper respiratory infection (kitty cold). If he's a stray and hasn't been to the vet yet, you don't know if he's vaccinated, so I'd keep him separated from your other two cats for the time-being in case whatever is causing the sneezing is contagious.
Kudos to you for welcoming this furry boy to your family and arranging for him to get a vet exam and neuter surgery! I hope all goes well and the sneezing proves to be nothing serious!
Cher
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