"I only have about 16" from the toilet sweep to where it attaches to the main waste line (running horizontal). That allows me to use a long sweep 3 x 3 x 3 long sweep wye, but no additional 3 x 3 x 2" vent t to vent the toilet separately."
You don't get out of it that easy. The toilet must be vented. If you have to take out more concrete, then you will.
If you need more room, you can put the wye and a 45 farther downstream.
This way you can throw in a rolled 3"x2" wye for the vent remembering to use waste fittings below the flood level of the fixtures.
I would not wet vent the lav into the shower.
Run them seperately, the vent for the shower can revent into the lav vent at 42".
If you decide the wet vent the lav with the shower, keep the vented portion either vertical or at a 45%. You could run the 2" line under the plumbing wall to achieve that and then "arm" over to the center of the shower pan.
One way to to do this would be to wye off sooner and run a line under the plumbing wall for everything. The toilet would wye off again from that line with a rolled wye fitting for the vent. The lav and shower would run under the wall too until the shower was wyed off and vented. If the lav was wet vented over the shower, then it would run straight from there until the lav was pulled off and then you could arm over for the shower trap.