The expression you have would only find keyword repeated three times with no intervening characters. Why are you trying to do that? You might be able to use the ranker, and turn up likepdocfreq, or in theory you could do the following:
keyword keyword keyword /.
Which if you allow post processing will find the keyword 3 times, although as it is doing post processing the performance may suffer.
I may not have been clear, but setting likepdocfreq to 1000, and all the other rank knobs to zero, and searching for LIKEP 'dell' will return you results ordered by the number of occurrences of dell in the document.
Doing the query:
dell dell dell /.
while allowing post-processing and REX in the query via query protection will find documents containing dell at least 3 times.