Basement plumbing rough-in


I decided to install basement bathroom.
Installed new basin, run 3" PVC pipe for toilet, tub, sink drains ensured 1/4" slope per foot, filled with water no leaks everything is a OK. Ready to concrete
Oh wait, there is a leak, where 3" pipe enters basin...
So question would be how should I sealed pipe that enter sewage pit?
( I know if pump is working water level should never get that high, and yet I feel need to seal...)
Thanks, Brian
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Is the fitting that comes with the basin, for 3" or 4" ?

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Just 3" hole on the side of the basin....
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Is it just a hole, or is it a fitting or rubber bushing?

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