Hmm, this is a fun question. If I could only keep 3 of my primes, my answer would be different because I have so many. I'll never part with my 24L. I will part with my 35L and 50L if I buy the new 24-70L II. I won't part with my 85L, nor my 135L. Going beyond that requires more specialized shooting, so I'd say I'd keep a 24L, 85L, and 135L. Expensive lineup for sure, but let's just pretend you could get them at Walmart.
Out of all of my primes, I'd keep the 24L, 85L, and 200L. IMO, the 300 and up lenses you just can't count, because anybody needing those will already be so specialized. If a 200-400L zoom lens comes out though, boy that would be interesting.
Can I just buy 3 of the 24L's and count that?