Re: Hello，teachers！ Which sentence is correct？Thankyou!
An exercise is one type of activity. Push-ups are one type of exercise. Sit-ups are another type.
Exercise as a non-count noun refers to any type of thing you do that moves your body around.
I don't like to exercise is a general statement.
I don't like to do this type of exercise is a specific statement.
#1 is probably what you want to say about someone who doesn't move around much. #3 has about the same meaning, but I don't like it as well.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.