Minimum period of a signal
It's already answered in the comments so just to close it out: the answers are 3, 7 and 3. The periodicity is basically given by the denominator of the fraction in front of 2*pi*n after the fraction has been simplified (no common divisors between nominator and denominator).  The simplified fractions for the cases are 1/3, 5/7 and 7/3 so the periodicity is 3, 7 and 3 respectively.