At first I thought the thick one is like a spacer to keep the carb cool and the thin ones are the real gaskets.
Is the thick one ok with the whole "keeps the carb cooler"-thing in mind?

Should be fine.

just the thick one or all three. I don't get that there are two thin ones.

Another thing is the bowl vent.
Until now it wasn connected and sealed with a screw in a hose.
I have that charcoal thing in the front (which seems fully connected).

Is it critical to connect the bowl vent somewhere?

Just the thick one. never stack gaskets. Bowl vent is for emissions, so fuel vapors don't escape into the atmosphere. Hook it to the charcoal cannister if you want.

Just remember if you hook you carb bowl vent to the canister you need the canister system working so the vapors stored in the canister get purged out. You dont want the canister to fill with fuel vapors and not have them purged out or then the vapors will back up and make it hard to start on hot soaks and flood real easy. Ron

I can not guarantee, that the system and/or the canister work porperly.

Is it ok to leave the vent disconnected and sealed?

So now I am finished doing the rebuild. Took a bit longer, because I had to seal two cracks in the bowl and seal the primary wells.

At first I did tuning by ear to get it streetable. Then drove to a shop with an O2-sensor.
That thing only shows Lambda not air fuel ratio.
Although I turned the idle screws simmetrically, the right bank ran richer.
Now I have both banks on 0,91 Lambda. And when opening the throttle, it holds it there almost all the time.