I can’t connect my Apple Watch 3 to my new iPhone 11

my grown children brought my a Apple Watch 3 a couple of years ago and now I went from a iPhone 8 to an 11 and I can’t connect my Apple Watch to it but b for I got the iPhone 11 my apple jst kept jst shutting off on me so nw I don’t what to do I’ve even remove it reset it and it still not working and I took to geek squad any help

[Re-Titled by Moderator]