There are three colours on a tricolour.

Because those three colours can mix together to create other colours

Im Not So Sure About Colours On A Monitor Screen But..

Humans Can Only See Three Colours, Green, Blue And Red For Example A Leaf Looks Green Because Red And Blue Light Are Being Absorbed But Green Light Is Being Reflected. The Three Colours Mix To Create Other Colours Such As Magenta Or Cyan. If Something Looks Black Its Because All Three Colours Are Being Absorbed If Something Looks White Its Because All Three Colours Are Being Reflected

Primary colors

the three colours are red blue and green. these are the only three colours used because they are the primary colours and if you mix them you can get a whole range of coulours. [=