Firstly, yes you will need to buy a new license for Construct 3 as it's a completely separate program. However, if you already have a Construct 2 license, you are entitled to a discount. All offers for Construct 3 as well as how to get any available discounts can be found here.
Secondly, to put it simply, seats refer to how many people in an organisation require the use of the software. But if you're buying a personal license this won't apply to you.