The Y double slit is actually pretty easy if you think of from a geometry perspective.
You have two slits in a barrier that is some distance from a screen parallel to the barrier. If you draw a line from the middle of each slit to a point on the screen that is exactly in the middle, then the distance is the same from the slit to that point on the screen for both pathways. That means that two waves starting in phase at the slits will hit that point at the same time and therefore in phase. The amplitude is doubled. Now take that same drawing and instead of picking the point in the middle, pick any random point on the screen. If you draw two lines, one from the midpoint of each slit to the random point on the screen and compare the distances, you can predict what type of interference you get. If the lines differ by a multiple of lambda, the waves are in phase and you get a bright spot. If the distances differ by some multiple of lambda + 1/2 lambda, then the waves are perfectly out of phase and you get destructive interference.
Geometrical logic says that the random distribution on line pairs should hit the screen in such a way where they differ by gradually more as you move away from the midpoint. This means that the difference goes from 0 to 1/2 lambda to lambda to 1 1/2 lambda, etc... The screen gets spots that go from bright to dark back to bright back to dark until you run out of space on the screen.
A pisture really helps. Look at any phsyics text and draw lines from the bright spots to the middle of each slit and the difference in distance will always be a multiple of lambda.