Im installing a new bathroom so I cut the concrete to get to the existing 3in waste line. My toilet and vanity are side by side and Im wondering if I can I tie my sink drain into my toilet driain berfore it gets to the existing 3in line.

Your toilet needs to be vented (2" vent), so, in some states, you can't connect the sink into the existing toilet drain if it connects BEFORE the toilet vent connects to the toilet drain line. With that being said, in other states, as long as you connect into the toilet drain AND roll a 3"x2" wye above the centerline of the toilet drain then you may be able to use this new sink drain and vent to WET VENT the toilet (sink drain and sink vent must remain 2" all the way until the 2" vent connects into a 2" or larger vent).

Check to see if wet venting is allowed in your state...post back with answer, OK?

Mark