Bathroom, dryer and central vac - Can any of the exhausts be combined, and can any use the same penetration?
If you have a gas dryer, that would be a no for the dryer, no question. For electric dryer and the other devices, it feels to be against code to me, though I can't find a specific citation on a quick look. In any case, there is a risk for one device to back feed another device unless all devices have effective backdraft dampers on the device itself. I doubt dryers and vacs have such devices, and for the ones I've seen on bathroom fans, I would not deem them "effective".

It's not a good idea. Is there any way to get enough room by transitioning into equivalent rectangular ducts ganged together? It may take a custom built fitting, but you need to exhaust through a combined equivalent area of all 3 ducts added together whether combined or not. So if there's not enough room for this, it's not going to work anyway.