"行きたがる" is used in the third person and is not a 100% certainty. It is actually "行きたい", but "行きたい" is 100% sure.
Use たがる when expressing the conclusion that the speaker draws from other people’s behaviors, remarks, etc. If someone talks about how interesting a place is and how he likes it every day, then you can use this sentence to express that you think he wants to go there. Generally don't use this for yourself.
I want to go. It can be used for yourself or anyone else. Used in the second person, it usually appears in the question. Used in the third person to indicate that you are 100% sure that he wants to go. For example, he told you that he wanted to go to a certain place, and you can use this when you turn around and tell others.
In the N3 test, you can only choose "行きたがる" for the third person.