I woud like to use SplitAt() to freeze the 3rd column, while columns 1,2 and 4 are Hidden isit possible to do that ?

Thanks for your help

Latest stable release will not support such functionality ( hidden columns in left part of split )
If you really need such functionality - you can contact us directly at support@dhtmlx.com for latest version of related extension which will work correctly in described scenario
But you can just move all hidden columns in right part of grid, in such case it will work correctly with current version of grid.