How do I get control back from a 3 year old?

I am only seventeen but I have a cousin who can act like your daughter sometimes and it is hard to take care of her sometimes. What I do to my cousin is I talk to her in a stern voice and make eye contact. The eye contact is the key it shows authority don't force her to look at you just say her name until she looks at you even if it takes about a hundred times before she does. I am one of the only one my cousin listens to and I never lay a hand on her, yell at her, etc. Don't spank your child because physical abuse like that doesn't help much you just have to have patience. Also when she goes into a tantrum just put her in her room and ignore her until she calms down then talk to her. I hope this helps!