My UK visitor visa interview is on 18 January and i am planning to visit in June, 2018. I saw a PDF stating i cannot apply 3 months before my intended date to visit UK. Will they refuse my application?
You cannot apply earlier than three months before you travel. See Standard Visitor Visa on the UK government web site. Scroll down to "How long will it take?"
I don't know whether you'll be refused, or if they'll just return your application to you with a note asking to to apply later. If you've already applied I guess you'll find out soon enough.