Hi Alex,
If you fire one electron at a time and have one slit, you will get the electrons building up on the screen behind the slit, as one would expect, but even here the first hint of quantum wierdness begins to display itself. If you continue to fire one at a time through the one slit, the pattern begins to show diffraction as Bert said ... how can individual particles, fired one at a time, produce diffraction which is a property of waves ..... aren't our electrons particles ...???
If you have three slits, fire your electrons one at a time, you will get your interference pattern ... once again a wave property .... hang on, not only are we firing particles we're doing it one at a time. Even if they do exhibit wave properties how can we get the interference pattern when we're firing them one at a time, their "wave properties" can't interfere ... but they do
..... or should I say the electron goes through all the slits and interferes with itself.
Freaky stuff ....
Andrew.