The structures of three 3-methyl-1H-1,2,4-triazole-5-thione derivatives are reported. The structure of 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione, C3H6N4S, (I), has been redetermined with an improved model for the H atoms: the non-H atoms of (I) all lie on mirror planes in space group Pbcm, and the H atoms of the methyl group are disordered over two sets of reflection-related atomic sites having occupancy 0.5: two independent N-H center dot center dot center dot S hydrogen bonds link the molecules of compound (I) into complex sheets. The non-H atoms in the molecules of 4-[(E)-(3,4-dimethoxybenzylidene) amino]-3-methyl-1H-1,2,4-triazole- 5(4H)-thione, C12H14N4O2S, (II), despite lying in general positions are close to planar, with a dihedral angle between the two rings of 6.31 (10) degrees : the molecules of compound (II) are linked by a three-centre N-H center dot center dot center dot (O)2 hydrogen bond into a C(10) C(11)[ R12 (5)] chain of rings. A second polymorph of 4-[(E)-(5bromo- 2-hydroxy-5-bromobenzylidene) amino]-3-methyl-1H-1,2,4-triazole-5(4H)thione, C10H9BrN4OS, (III), has been identified; the non-H atoms are nearly coplanar with a dihedral angle between the two rings of 1.9 (4) degrees. There is an intramolecular O-H center dot center dot center dot N hydrogen bond and the molecules are linked by NH center dot center dot center dot S hydrogen bonds, forming centrosymmetric R-2(2) (8) dimers. Comparisons are made with some related structures.