Graph 1 is not a function because in some places, it has two y values for each x value. Graph 2 and 3 are both functions.
A good way to test for functions and non-functions on graphs is to use the "vertical line test." If you draw lines parallel to the y-axis, those lines should only pass through the graph ONCE. If at any point the same vertical line touches the graph twice or more, then it fails the test and is not considered a function.
