Plymouth St Andrew marriages:
18 Aug 1838 Walter SAWYER, widower, father also Walter SAWYER; married Mary PARSONS, widow, father John HILSON (or HELSON can't decide). Witnesses: Sophia PARSONS and James BOULTER.
Holbeton marriages:
9 Nov 1798 James PARSONS, sojourner in this parish, married Mary HILSON, spinster of this parish.
Newton Ferrers baptisms:
29 Nov 1777 Mary HILSDON dau of John & Agnes of Revelstoke.
19 Nov 1775 Jenny HILSON dau of John & Agnes (so spelled both ways)
27 Jul 1783 Joanna HILSON dau of John & Egnes of Revelstoke
Revelstoke baptisms:
4 Nov 1785 Richard HELSON son of John & Agnes (This Richard married Catherine HEYDON in 1815 at Stoke Damerel. A witness at the marriage of George HILSON & Nancy STOYLE in 1822 was Cathrine HILLSON)
Revelstoke burials:
8 Mar 1844 Agnes HILSON aged 92 of Noss. (She is shown in the 1841 census as Agnes HIBSON living with Andrew WALKE and his wife Joannah. An Andrew WALK married a Joanna HILSON at Stoke Damerel 12 Jan 1810; wit: Rich & Jane HILSON)
So I think Mary only had 2 husbands. She was born a HILSON, married James PARSONS and then WALTER SAWYER.