Re: Rough in question?
Depending upon the distance I would be more inclined to directly connect each line to the horizontal drain. This would allow the tub/shower to operate if the 3" line from the toilet became plugged. I usually either make a direct connection into my vent or the toilet and Wye off for the other one. If the lav is close enough I will tie the lav drain into the vent stack to wash the base of the vent. Then if there is less than 5' from the vent I will tie my tub/shower right into the horizontal drain.
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