it, best to use a sani-t and a p-trap, with a vent coming off the top of the sani-t. Even if you just use an air admittance valve, and dont run a full vent pipe to outdoors, its much better than an un-vented s-trap. The s-trap will siphon and loose its seal under the right conditions that's why they are forbidden by all plumbing codes. That 3" waste lateral will most likely have big plugs of waste going by, which will have the pressure to easily blow out or siphon a 1 1/2 trap.
It may be that the PVC near by is vented already, and will make a better installation, so I just figured I would throw that out there.
That neoprene doughnut pictured is what you need to make the connection there if you choose to. You might have to combine two sizes to get from 3 - 1 1/2 though, especially if you are working from a big box plumbing isle and not a supply house. Home depot does stock the doughnuts though.
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