THREE AND ONE CONFIGURATION

Three and One configuration

The two highlighted cells draw your attention to the pattern cell containing a 3. This cell requires three lamps, but has four vacant adjacent cells. We need to determine which of the four will contain lamps. The diagonally adjacent pattern cell containing a 1 gives us the answer. Of the two cells C6-R4 and C7-R3, only on is permitted to contain a lamp, thus forcing the two highlighted cells to contain a lamp.