The 2 questions i have are:
Yes, a wye fitting is the right one for horizontal venting.1)-Can I vent off the 3" horizontal drain line in the attic area with wye while the drain is still running horizontal, or do I have to do the vent connection when the drain makes it's vertical drop. The vent part of the wye would be pointing up. It would make for a cleaner install to the vent connection in the attic area instead if inside the finished laundry area.
If you were venting on the vertical, are as it turned vertical, then it would be a santee fitting.
A medium 90 is fine below the flange.2)-Is it ok to do a 90 degree turn out of the flange? There are 8" wooden I Beams as joist so I don't have alot of room for a more gentle bend (I plan on using a long 90 el there.) The joist run parallel to the 3" drain where it attaches to the flange and where I will punch through one, it will be above a supporting wall below.
It's doubtfull that you have have room for more then that, unless you drop below the floor joist.
You may want to check with the framing inspector on how large a hole through the joist.