You can keep your 3d vision sharp by setting up puzzle positions you find in books on your chessboard and solving them without moving the pieces. Writing down the solution before looking at it on the book is mandatory, and it'll also train your scoresheet maintenance attitude.
Mind that another thing which could be detrimental in solving puzzle on the PC screen is the ability it gives you to start moving before you figured out the whole solution, and thinking you solved it while you really didn't grasp the whole meaning of the puzzle enough to get the confidence to go for your partial solution if you were actually playing the same position over the board.
recently i have been playing and solving puzzles all on computer, ALL 2D !!
and now I found myself having problem when playing OTB, especially with the time control that the tournament offers... ( i tend to think very long, because im not USED to the large board and situation )
and my tactical sight also drops dramatically when playing OTB
what should i do now?? HELP ME PLS... im facing a tournament soon ( 25 mins per person )