Question 919961
The only trouble I can find is that the natural numbers start at 1.  The second version, if x itself is 1, would have one of the numbers to be summed, x-1 NOT a natural number, because 1-1=0, which is acceptable for WHOLE numbers, but not for NATURAL numbers.  


The first version, 3x+3, is acceptable.  Each of the three consecutive numbers will be Natural if x is chosen among the Natural numbers.