Couldnt you default the all the cells to "locked" and then give those three employees the pass word to unlock it? This would prevent the other 27 from editing the document.
Choose all the cells and then right click to Format Cells. Go to Protection and check off all the cells to "locked." Then go to Tools, the Protection menu should be a drop down and choose that. Check off your protection criteria and create a password. Now the document should be accessible, but uneditable to those 27. It is then only editable to you and anyone else that knows your password.