Ok Greg your off to a good start and as you mentioned in PM it's helped you bypass that initial "speed bump" so that's great!
In the footer, remember to paste my code examples exactly only remove the forumR division completely if you do not want the right column . Since your want just the left and then the right to be full width we will remove it and adjust the width of the main forum/threads etc to be full width so it's stretches.
Also the shorthread/postbit areas should not overlap the left "column" we have made see my demo shot here:
Now to fix the center column as we were calling it now it's pretty much the right column if you think about it regardless though change this:
And we do this to counteract what you ran into yourself... you can see in my screenshot it's fine in the area I showed however by taking out the right "column" as you did that makes the forums/threads/posts float left (we did tell it this to be fair ) so we bypass that by removing the width attribute and telling it to be 165px - you may want to adjust that, I initially did the two columns at 150px I'm sure by now you figured that out and adjusted the numbers just remember what I said about css - sometimes not defining somehting is better than over defining everything as some things are inherited regardless.
Now with the changes we have this:
Which carries through the entire site . *I did notice one misc issue if you have the forum sidebar enabled so let me know if your using that and if so we'll address otherwise this has you more on track and after some tinkering you should be good to go and in the least you now grasp css much better! Now that you have some of this under your belt check out this mod that does something similar.... https://vborg.vbsupport.ru/showthread.php?t=230329 and there's others so be sure to use the search and have a look around... with your new CSS abilities you should be able to understand that part of anything you run across now .