I'be been thinking for a while that the 'primes' in question here are in fact...
primary colours, prompted by the secret 'RBY' capital letters in the title (Red-Blue-Yellow).
With the latest hint, I think I see that the three '-fixes' here are actually...
the stages of a traffic light while a vehicle is waiting at a junction. (Certainly in the UK, at least...)
My ‘prefix’ is a ‘prime’
The 'prefix' (first stage) is RED, a primary colour and the colour of a stop light.
My ‘infix’ is a combination of ‘primes’
The 'infix' (middle stage) is RED and (what we'll call) YELLOW (but might more often be called 'amber') combined, the two primary colours that appear together in UK traffic lights to prompt drivers to get ready to move soon.
My ‘suffix’ is found between ‘primes’
The 'suffix' (final stage) is GREEN, the colour of the 'go' light, and which - in a rainbow - appears between yellow and blue, two primary colours.
My whole is often found at the origin of a very long sequence!
Traffic lights are often found at the start of a long series of halted cars!