WordNet sense 1 (tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed):

devil's weed, Tribulus terestris

WordNet sense 2 (a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits):

water chestnut, water chestnut plant

WordNet sense 3 (Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America):

star-thistle, Centauria calcitrapa