What is the name of the compound with the formula Pl3?

It would be phosphorus (I) iodide. However, this is not a stable compound; under normal conditions phosphorus and iodine combine to produce phosphorus (III) iodide (which is also not stable ... it's explosive ... but is considerably more stable than phosphorus (I) iodide is).

The electron configuration of the phosphorus in PI3 is [Ne]s2, while that of PI is [Ne]s2p2. The filled s subshell is at least metastable, while a filled s subshell plus a partially filled p subshell is normally not and requires considerable effort to maintain.