Let a, b, c represent the numbers of subscribers who watched 1, 2, and 3 videos, respectively.
.. a +b +c = 312 . . . . . . . there are 312 subscribers, total
.. a +2b +3c = 740 . . . . there were 740 total views
Subtract double the first equation from the second.
.. (a +2b +3c) -2(a +b +c) = (740) -2(312)
.. c -a = 740 -624 = 116
116 more people watched all three videos than watched one video.