Hi Sara,
I found some pictures that can help you:
1. We normally find benches in parks and gardens. They are designed for more than one person to sit on.
https://www.pexels.com/search/bench/
2. We use chairs in houses, offices, restaurants. They are designed for only one person to sit on.
https://unsplash.com/search/photos/chair
3. A seat is a part of a plane, train, car, cinema, stadium ect. or a piece of furniture (like benches, chairs and sofa) designed for people to sit on. We normally use the word seat in sentences like "Have a seat" or "My seat is 33C".
I hope you have a better understanding of the difference in meaning. Try to google those words yourself to see pictures.
Bye