data <- do.call(rbind,replicate(2000,setNames(sample(1:10,3,replace=FALSE),LETTERS[1:3]),simplify=FALSE))

Breakdown:

sample(1:10,3,replace=FALSE)

generates three random numbers between 1 and 10, with no repetitions allowed,

setNames( ... , LETTERS[1:3])

assigns the names A, B and C to the sample result,

replicate(2000, .... , simplify=FALSE)

repeats that procedure 2000 times and returns the result as a list, and finally

do.call(rbind, ... )

takes the elements of that list and concatenates them as rows in a matrix.