Royal Ancestry by Douglas Richardson Vol. II. page 611
ROYAL ANCESTRY by Douglas Richardson Vol. IV page 175
PHILIPPE MORTIMER, married (1st) JOHN DE HASTINGS, Knt., 3rd Earl of Pembroke [see HASTINGS 14]; (2nd) RICHARD DE ARUNDEL, K.G., Earl of Arundel and Surrey [see FITZ ALAN 12]; (3rd) THOMAS POYNINGS, Knt., 5th Lord Saint John of Basing) [see PAULET 11].
ROYAL ANCESTRY by Douglas Richardson Vol. IV page 323-324
Alison Weir, Britain's Royal Families: The Complete Genealogy (London, U.K.: The Bodley Head, 1999), page 97-98. Hereinafter cited as Britain's Royal Families.
G.E. Cokayne; with Vicary Gibbs, H.A. Doubleday, Geoffrey H. White, Duncan Warrand and Lord Howard de Walden, editors, The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct or Dormant, new ed., 13 volumes in 14 (1910-1959; reprint in 6 volumes, Gloucester, U.K.: Alan Sutton Publishing, 2000), volume I, page 245.
John Hastings married Elizabeth of Lancaster the daughter of John of Gaunt, in 1380, but the marriage was unconsummated (he was 8 and she 17 at the time of the marriage) and was annulled after she became pregnant by John Holland, whom she subsequently married. Sir Bernard Burke´s Dormant / Extinct Peerage, p267 notes he died jousting at King Richard´s Christmas Court in 1389 leaving no issue from his marriage to Philippa the dtr.of Edmund Mortimer who was also very young.
Some sources have it that Philippa was wed 5 times- Sir Henry Percy, 1364-1403, in 1380. Sir Thomas de Camoys, 1383-1421. Sir John Hastings, 1372-1381, in 1385. Richard FitzAlan, 1348-1397, in 1390. Sir Thomas Poyntz, -1428, in 1399. I am awaiting confirmation on this data.