#1 Can you install both versions on the same computer?
Yes
#2 Will the new Sonar Professional version overwrite the X3 version?
No
#3 Would I still have the ability to use all the plugins that are now available in Sonar X3 with the newer version?
You should be able to use quite a few, I am not sure about all, seeing that some have been updated in Patinum, such as TH3 if I'm not mistaken.
As far as Platinum over X3 , as someone said it's light years ahead, so the learning curve might be a little work for you , but worth it.