As what lucifer mentioned, the issue lies with your one-way valve. Instead of visualizing the water flowing out of the tank and into the main pipe, you should visualize it backwards, say main pipe water flowing into the tank outlet pipes (stopped by the one-way valve).

Its a piping system that says in case no pressure from mains, tank water pressure will take over. If got main pressure, it will top up your tank and bypass the tank water usage for outlet 1 and 3. Since one of the one-way valve is faulty, then system fails and it will flow main water into the tank non-stop.

One of the two valves is faulty but my best advice is to replace both to save time from testing which valve is faulty. Just go to any hardware shop and ask for one-way valve with the correct size (measure your pipe diameter). Also, my advice on replacing both the valves is to sumbat outlet 1 and 3 from inside the tank (with a thick cloth and must be tight) and off main pipe tap. Drain the water in outlet 1 and 3 (Just open any tap to outlet 1 and 3). Do valve replacement with two pipe wrench. Don't forget to apply fresh Plumbing tape (white tape).

Too bad my house tanki piping system isn't like that and whenever water shortage, all the taps connected directly to the mains will run dry.

Its a good way to test but considering risking more overflowing water from the tank, might as well get it done before your water bill sky rockets.

Goodluck