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In all three the phore bit is derived from the Greek "Phoros" (bearing) and its derived verb "Pherein" (to carry).
Chromatophores are cells carrying pigments, from the Greek "chroma", for colour.
Iridophores are iridescent or reflective pigment cells containing guanidine-based reflective platelets. "Irid" or "Iris" stems from the Latin but ultimately Greek word for rainbow.
Photophores are not cells, but rather light-producing organs found especially in marine animals that emit light from specialized structures or derive light from symbiotic luminescent bacteria. Photo stems from the Greek "Phos" or "Phot" for light.
If you want to understand this in more details, a good place to start is Messenger's extensive review on chromatophores http://www.cephbase.utmb.edu/refdb/pdf/6818.pdf
from there you'll find lots of references
he also mentions irridophores, leucophores etc...