My 3yr old daughter just fell and hit her on tiles very hard. threre is a huge bump, we applied arnica gel. should we be concerned?

Maybe, maybe not: Depends on details (bump location?). If a child is behaving differently after a head bonk, she should be evaluated by her primary doctor. Sometimes an emergency room visit is needed, depending on the symptoms. Head bumps in which there is low-impact on the head, little crying, no changes in behavior and no headache after the first hour, etc..., usually don't cause internal injuries.