  We investigate the effect of strong interaction in the dressed energy levels
of the two level emitters. Strong dipole-dipole interactions give rise to new
sidebands in the fluorescence spectrum due to specific couplings among the
collective dressed levels which in turn depends on the spatial configuration of
atoms. These couplings are the main responsible for the frequencies and variety
of sidebands. We explain the general method for finding the dressed energy
levels for a system of any number of strongly coupled atoms and we solve this
problem for two different spatial configurations of three coupled two-level
emitters. We show that the coupling among dressed levels and consequently
energies and number of sidebands in the fluorescence spectrum are different for
each configuration. Thus the fluorescence spectrum of strongly interacting
atoms contains information about the number and configuration of atoms.
