I am currently running 6 copies of Joomla 3 in a single hosting account and would love to make it just 1 copy! I have 3 sites which are live sites with no sharing of users (except for me as administrator) or content. There is also a test version ...

Only all menus and menu items can be shared.

Also, contents on some of the subsite may increase considerably, should I expect performance issues on all the portal if using a single installation?

Sure, it's same as running multiple sites.

... the websites? Also, contents on some of the subsite may increase considerably, should I expect performance issues on all the portal if using a single installation?

Please check last two case studies, you can have single Joomla installation:
https://alterbrains.com/joomla-extensio ... se-studies

Content will be same but displayed differently per site via site-specific menu items.

... (modules as well as articles), but for which the actual content can be different on each subsite. Please check this case study, you can have single Joomla website which is loaded via multiple domains: https://alterbrains.com/joomla-extensions/administration/multisites-manager/case-studies/subsites-with-minor-differences-in-menu-items-and-or-modules ...

Please check last two case studies:
https://alterbrains.com/joomla-extensio ... se-studies

Session can't be kept between such domains, but we cave single login.

... ⋅ Feature: new setting 'Inline Term CSS' allows to set CSS rules for terms highlighted in text. ⋅ Fix: parsing of terms with single quotes, e.g.  O'Neill . ⋅ Feature: EasyBlog support. Read full article...

... a grid with each category cell having its own background image. We also talked the customer into scaling back on the category hierarchy to allow a single phpBB install. I'm not sure if we are going to need Mightysites for this particular job but will be licensing a copy to play with. It should ...

MightySites doesn't affect Joomla routing.

You can combine multiple sitemaps into single:
https://support.google.com/webmasters/a ... 5712?hl=en

... for a domain and associated subdomains. I could not find one but did find a company with a script that could be run as a cron job to generate a single sitemap for both the domain and any number of subdomains. Link is below: https://www.xml-sitemaps.com/news-20131119.html If MightySites can make ...