Depends on what subject
In a science, they will generally encourage you to write more reasons that given and will only take marks off if you write something incorrect. If you are unsure about the third reason leave it out. If you are certain about all three, then it's not a bad idea to put it in.
In a humanities subject, if its like "Discuss two reasons..." you should just stick with the two, otherwise you will likely struggle to get the depth they want