2pp is when your eye level is somewhere in the middle of an object.
3pp is when you're looking down or up.
2pp won't let you see more than 2 sides of a box
3pp will let you see 3 sides of the box.
2pp usually seen on buildings on street corners, where you can see both streets but your eyes are not below the base of the building or above the roof.
3pp I would say it's not worm's eye view because the worm would still share the same ground. it has to be a lower level than the object, so you usually see this when looking down from a plane, or up from let's say a ditch.
2pp can look exagerated when you feel that the streets on each corner are too long
3pp can look somewhat flat and give a sense of detachment/distance.
they BOTH can happen within the same room... a book resting a little outside the edge of a high shelf or you sitting on a chair and looking down at your computer case which is on the floor or a small garbage basket.
when you feel like you can better handle 2pp and 3pp start looking into camera lenses and how they affect the way we see things. think of the distortion of people's faces when you see them through the peephole for example.