1 About Heyy
You can write either hey or add two more y to it. It is an internet slang which is a flirtatious way to say ‘hi’. It also means Hey you and is the most common definition used on Snapchat, WhatsApp, Facebook and Instagram.
2 Difference between hey, heyy and heyyy?
The most common of them is hey, which means "Hello how are you doing?" If a girl sends you a message stating hey it means that you are full-blown friends and share a great bond with each other.
Adding an extra y to hey which makes it heyy can also mean “Hi, I am somewhat interested in you” It means that she wants to make the first move towards you and shows that she is interested in you.
Now, if a girl sends you heyyy then it means that she is screaming out loud to ask her out for a date. It can also mean that she is interested in having casual sex with you, and you should ask her for it as she would not say no because she herself is giving you the indications.