First post newbie!

I have a URL based on the name of my company. I recently built 2 other sites using URL's with the main keyword people use when searching for my service. Each site is completely different, as far as how its structured, and the wording of the content. Main URL is ranking number 1 in all the cities I market too. Question is; should put links on all the sites to tie them together, or leave as standalone?

Thank you