Please note that I have continuously deferred to the pro plumbers for legality of setup, etc. I was merely positing the question of whether it would in fact siphon or not. I don't know exactly how a toilet flush goes through a 3" line, but
IF, and only if, it didn't have enough water passing at once to completely cover the 2" inlet from the tub and the 2" wet vent from the lav, it probably wouldn't siphon in this setup. If that is not the case, then the vent will not help at all.
As I've said before, I personally don't believe in wet venting anything, so I never do this setup... I always vent every trap individually in my installations. I don't know a lot about proper wet venting b/c I don't ever do it or allow it to be done by plumbers on my jobs. I'm not sure, my county also might not allow any wet venting other than a toilet venting off a stack (seem to recall something about that from one of my plumbers), so its just not something I deal with.
In thinking through what the plumbers have said, and how blazingly fast today's toilets drain out 1.6 gallons compared to the slow draining of the old guzzlers... that would pretty much have to fill the 3" line high enough to siphon the trap. So, I'm in agreement with the pros that this setup will become unsafe if you don't leave the water running in the tub at all times to refill the trap
For the record, I fully believe in the concept of code. I think that it should be followed any time it is reasonably feasible to do so. I don't necessarily believe in the current system of enforcement, I've seen too much corruption in it, and there are too many problems with it for me to say that I think its a good system these days. However, I do advocate doing things to code whenever possible. If a situation like this comes up where getting it to code is quite difficult, and there is a practical solution that is functional and safe, I'm ok with going slightly outside the code. Agree with me or not, I'm not here to convince anyone, but I deal with stupid code crap every day that makes no sense, and have accepted that sometimes you're better off doing it the right way in the circumstance than blindly following the code. That said, 99.9% of my work is up to code, there are just a few odd circumstances where it makes sense to do a good solution that might be outside the general bounds of code. Given what's been discussed here, I don't see any good way to do this properly w/o digging it out and doing it up to code if there is indeed a trap down there.
Was this rough-in inspected? Do you have documentation? I believe you have a claim against the inspector, as well as the plumber who pulled the permit for the inspection, if it was passed in this form, but maybe the more experienced plumbers can chime in on that as well. If it wasn't inspected, you're on your own for the repairs.