Drum fill generator

Jelly, 15 bytes

“LRK”ṗ;`E3Ƥ$ÐṂX

A monadic Link accepting an integer which yields a list of characters.

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How?

Pretty similar to Nick Kennedy's Jelly answer...

“LRK”ṗ;`E3Ƥ$ÐṂX - Link: integer, n
“LRK”           - "LRK"
     ṗ          - Cartesian power (all length n strings with alphabet "LRK")
            ÐṂ  - keep those entries which are minimal under:
           $    -   last two links as a monad:
      ;`        -     concatenate with itself
         3Ƥ     -     for each infix of length 3:
        E       -       all equal?
              X - random element

05AB1E, 15 bytes

…LRKãʒDJÅγà3‹}Ω

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…LRK                # literal "LRK"
    ã               # strings made of `input` of those letters
     ʒ       }      # filter:
      DJ            #  concatenate the string with itself
        Åγ          #  run-length encode
          à         #  maximum (largest run length)
           3‹       #  less than 3?
              Ω     # get a random element

R, 67 bytes

n=scan();while(max(rle(c(x<-sample(c("L","R","K"),n,T),x))$l)>2)0;x

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A full program taking an integer, n and implicitly printing the drum pattern.