There are #3xx7+5=26# valence electrons to distribute, i.e. #13 " electron pairs"#.
Explanation:
Around EACH bound #Cl# atom there are 3 lone pairs; there are #3xxP-Cl# bonds; the thirteenth lone pair resides on phosphorus: #:P(-Cl)_3#. Since there are 4 electron pairs around phosphorus, the geometry is based upon a tetrahedron, but since one of these electron pairs is a stereochemically active non-bonding pair, the geometry around phosphorus is described as trigonal pyramidal.