The three-letter route
ZIP is the three-letter answer. Trace ZIP from row 5, column 4, then move up to row 4, column 4 and right to row 4, column 5. The path reads Z → I → P. It begins on the bottom row, makes one turn, and finishes on the right edge, which gives you a compact route to confirm before you start assigning longer words.
For a position-by-position check, use: 1 at row 5, column 4; 2 at row 4, column 4; 3 at row 4, column 5. The first cell has one open orthogonal neighbor, while the final cell has two. After ZIP is assigned, 15 open cells remain for the other three answers, so it is a useful first route without giving away the rest of the board.