[3 5 7]
[7 5 3]
[8 4 2]
[5 7 3]
[1 2 3]
[1 1 1]
[0 1 2]
So after the tests listed above I felt confident enough to guess.
At that point, I correctly answered the question.
I was relieved, in fact, when it worked with negatives and floats in a "safe" range.
I also tested with 1,1,2 and 1,2,2 to make sure that the required increase applied to ALL of the values, not just a specific pair.
Like others here have said it wasn't a particularly hard "rule" to figure out. Easy to immediately rule out geometric relationship as in 2^y, which didn't leave a whole lot of possibilities to test. For the commenters here, I'd attribute ease of finding the solution to familiarity with the kinds of problems that programming presents.
Which leads to the idea there's value in learning even the rudiments of programming. Logically, it should encourage better problem-solving skills in general. We might think there this has important implications for our educational systems. But I know, that's probably not realistic at all.
4,6,8 Y 1,1,1 N 1,2,3 Y 1,6666,8777 Y 1,0,1 N 3,2,1 N 3,2,3 N 5,6,4 N 7,5,6 N