Three reed switches are at the heart of the circuit, one fitted to each door. They close when a door is opened. An associated LED lights when a door is opened. The remainder of the circuit is powered by either D1, D2 or D3. However the 555 is not enabled until pin 4 goes high and this requires the output of either IC1a or IC1b to go high. In turn, this requires either pin 1 ,2 or 4 go high and this happens when a door opens. Because the high on each pin is only momentary (i.e. about 1/3 second, while C1 ,C2 or C3 is charging) there is only a short burst of buzzer activity (two brief beeps) at each door opening, after which it goes mute again. So the beep calls attention to the fact that a door has opened and the LED indicates which door, staying lit until [i door is closed. If another door opens before the first door is closed, there is another
beep and another LED lights.
 The circuit above is too complex. It is very poorly designed. The 3 signals diodes are doing NOTHING. The are simply across each other!! One diode could be placed in the supply line to the 555 if it is needed for the reset line to work correctly. R7 is not needed as the output is taken to the pins 2&6 and the 74LS32 chip can be replaced by 3 x 470u electrolytics. It can be simplified to this:

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