If you display by Album artist, you will see all the albums with "various artists" in the album artist field grouped together, and all the tracks that have the exact same album name (along with some other fields) will be grouped together in a single album. If you display by Artist, you will see all the tracks by an individual artist grouped together in one album if they all have the identical album name (along with some other fields). Individual artists must be separated by semicolons, such as "Bill; Joe" - if you have "Bill and Joe" or "Bill, Joe", then you have defined one new artists, not two existing ones.
Of course, you have MANY other display, sorting and grouping options available to you in MusicBee, and we have absolutely NO WAY of knowing what you are doing based just on your cryptic question.
One more thought for you - you say you are using MP3Tag, which implies to me that you are changing tag fields outside of MusicBee. What version are the tags being saved in? Does it match your MusicBee tag preferences? Have you tried re-scanning all files to make sure MusicBee has had a chance to see all the tag information that you may have changed behind its back? As a start, I would suggest you use the MB tag editor to review the current tags that it sees and compare them with the tags that you think it should see. Personally, I do not use MP3Tag with MusicBee, nor am I experienced enough with this program to know exactly how this might affect things, but with more information, I am sure others who do have that experience will respond to you.