Sorry - i'm back.
The circuit works fine when one switch is pressed after one another distinctly, it's fine. One switch triggers two light bulbs, the other one does not work.
The problem arises when I press two switches at the same time. This makes the two "contestant" light bulbs work at 3 volts, while the main light bulb takes up 6 volts. The only reason I can see for this is because the SCR gates trigger slow enough that two of them go at the same time, closing two gates.
Then I tested again, but when I press the first button on it's own, before holding it down and pressing the second one.; The same problem arises.
I tested the Zener diode - it seems to be working perfectly. Any ideas?
Further clarification:
When no light bulbs are pressed, the current going through the Zener diode is 36.2 mA. The current required for the SCR i'm using to open is 0.3 amps. When a push button is pressed, the current going through the Zener drops to nill, as expected - even when I hold a switch down. However, while holding this switch down, if I press another push button for another contestant, that light bulb lights up without any change in the DMM used to measure current going through the Zener.
The SCR i'm using is CRO2AM, while the Zener is a simple 9v 1A.