1.  ptochos = destitute, and in want: always rendered "poor" :  except in Luke 16:20, 22 (beggar); Gal. 4:9 (beggarly); Jas. 2:2 (poor man).

2.  penes = poor, as opposed to rich.  Occurs only in 2Cor. 9:9.

3.  praiis = meek, as distinguished from passionate.  Occ. only in Matt. 5:5; 21:5; 1Pet. 3:4.

These words are used in the Septuagint interchangeably for the same Hebrew word; but the contexts show that they are all used for the same class, viz., the fellahin, or poor of an oppressed country, living quite lives under tyrannical and oppressive rulers; and suffering deprivation from tax-gatherers and lawless neighbors.