There should be a vacuum diagram as a sticker under the electronics flap (where your ECM is).
Basically, the 3 plugs are for: 1 will have a rubber Y-splitter, that splitter sends vacuum to the fuel pressure regulator and the vacuum solenoid on the front of the engine (under the plastic cover). One of the lines from that solenoid goes back to that same area and connects to the EGR valve (it's at the very back of the engine, mounted directly to the intake manifold). Another line coming off that solenoid goes to the secondary air injection check valve (it's close to the solenoid, mounted near the exhaust manifold).
Back to those 3 plugs on the intake manifold, you now have 1 that is accounted for. Another line goes from the intake manifold to the electronics area, where it plugs into a plastic vacuum splitter with many plugs and lines going to it. That then powers the rest of the vacuum accessories on the car (folding headrests, central locking, HVAC actuators, vacuum reservoir, and whatever else needs vacuum). If you see an open plug on that vacuum splitter, don't worry, it's not leaking, any open port is automatically shutoff internally. However, that plastic splitter is known to crack and cause a vacuum leak, so inspect it for that, particularly at the ball bearing.
Back to those 3 plugs on the intake manifold, you now have one plug left. That line goes to the PCV (positive crankcase ventilation) solenoid, which is located under the windshield fluid reservoir.
So, how to get your car back up to its former glory? First, replace that leaking fuel pressure regulator, that's what is causing your rich fuel problem. You need some circlip pliers to remove it, but otherwise it's really easy. You just remove the circlip and pull the regulator out, pop the new regulator in, and reinstall the circlip.
Next, reconnect all your vacuum hoses, and do a general inspection of all the vacuum lines in the engine bay. After 21 years all the rubber gets stretched and starts to leak. Go to your nearest dealership, they sell the rubber vacuum line crazy cheap, I bought 3 meters for 20$. You can use that rubber line to replace any rubber lines, and make yourself new straight connectors for the hard plastic lines (the plastic lines will be fine). Elbows and splitters will have to be bought, but I'm sure they are also super cheap.
With all that done, you should be good to go! Your probably thought your car was fine during those few weeks because you had just enough vacuum leaks to offset the unmetered fuel leaking past the regulator. Anyway, it will make a world of difference once you do all that. The C220 isn't a slow car, it's just usually they have some age-related maintenance that needs to be done to get them back running strong.