It's hard to answer completely because we can't see your plumbing layout.
More than likely, your venting is not complete. How is the lav vented and what size? Does any other drain lines enter the 3" on upper floors?
What would really be helpful is if you uploaded a couple of pictures. Also, your location (state) would help us determine which plumbing code was used for your install- venting requirements vary per area.
When posting in forums, letting us know your location will help others give better feedback/advice/solutions to your questions
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