ION: a three-cell route with one turn
Trace ION through three positions: 1 at row 1, column 5; 2 at row 2, column 5; 3 at row 2, column 4. The letters read I → O → N. Start on I, move down to O, and then turn left to N. That gives the required three-letter spelling without adding an extra cell or making a diagonal move.
This path has one turn and two straight segments. It spans two rows and two columns, visits two edge cells, and includes one corner cell. The starting cell has two open orthogonal neighbors, while the ending cell has three. Once these three cells are assigned, three of the board’s 18 open cells are covered and 15 remain.