Can I do my graduate studies in Computer Science with computational physcis bachelor degree?
The best advise I can give you is to check what the prerequisites for graduate studies in computer science are. Check the homepages of the programs of different universities in the USA or Europe.

Some might require a degree in computer science or a related field, other just a certain amount of points related to computer science. 
To find out if a university considers computational physics as a related field you have to ask.

I am European, so I can't tell you if a GPA of 3.1 is good enough or how it affects your ability to get into a university in Europe.

You have to check and find out yourself. 
After graduating you should be able to do that.