It looks like you have number 1 correct.
For number 2, what is alike is the exponent of 2. To combine them, add them together. You can do this because they are like terms (aka have the same variable and exponent)
3-4-1= -2. The answer is -2x^2
For number 3, those are not like terms. They either do not have the same variable (x or y) or they do not have the same exponent (1 or 2)
Both the exponent and the variable have to be the same for it to be a like term.