Is it OK to have shower and lavatory drain past (enter upstream) of the toilet on way out to main sewer leaving the house? If so, will that require me to vent each fixture to keep toilet from sucking the traps behind it dry when flushed?

Each fixture that has a trap MUST be vented. It doesn't make any difference whether it's up or downstream from a major fixture. However, if wet vents are allowed in your area you can connect the shower drain to the lavatory drain and let the lavatory vent "wet vent" the shower. Otherwise both fixtures must have a vent. Good luck, Tom

Excellent, I can wet vent here so as long as I vent the toilet separate and wet vent shower to lav I'm covered?

In my arera you don't even have to vent the toilet. One 2" lavatory vent vents the entire group. Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic..
The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim. Good luck and thank you for rating my answer. Tom