Splitting a DB into multiple files
You really do not have much choice because how proportional fill works in SQL Server. 

Proportional fill and how to check whether it works

I am assuming you currently have one file in primary filegroup.


You can rebuild objects with clustered index and those will go to
new files totally because of more free space in newly created
tables.  As your current file start freeing up space you might not be able to move to new files any more (depending on free space in each file).
With DBCC SHRINKFILE only way this can work is with EMPTYFILE
option because you cannot shrink smaller than your current used
space. If you try to shrink with smaller size it will not work. 
Once you shrink with EMPTYFILE option current file will only hold
some system objects and rest will be split over to your 3 new files.


Next problem will be subsequent data inserts will use the old file (currently emptied) because of proportional fill algorithm. 

Here is an article by Paul Randal on the same topic.