If it is upper respiratory, be sure to get treatment and clean the food and water dishes daily with a diluted solution of bleach. This will help to keep it from spreading to others or from reinfecting. I wound up with 6 of them getting it, but not at the same time...a couple one week, two more the next, like kids with chicken pox(!), so that I finally had to make a meds chart just so I could keep it straight who'd had their shots/pills and how often, etc.!
It could just be a bit of catnip or something dry and dusty caught up the nose, but if it's gone on for 3 days, even that needs to be checked. Out here they were warning about foxtail seeds getting up the nose, in the ears, ingested or burrowing under the skin this summer. That can cause very serious problems.
I've got one cat who has an auto-immune disorder which makes her mouth very red and sore from time to time. Eating irritates the back of her throat and sinuses which makes her sneeze after every meal, even when she's had meds to alleviate the mouth thing.
Good luck and good health to your wee moggie, Bobert,