We present an analysis of bulk and surface electronic structures of thallium based ternary III-V-VI\(_2\) series of compounds TlMQ\(_2\), where M=Bi or Sb and Q=S, Se or Te, using the ab initio density functional theory framework. Based on parity analysis and (111) surface electronic structure, we predict TlSbSe\(_2\), TlSbTe\(_2\), TlBiSe\(_2\) and TlBiTe\(_2\) to be non-trivial topological insulators with a single Dirac cone at the \(\Gamma\)-point, and TlSbS\(_2\) and TlBiS\(_2\) to be trivial band insulators. Our predicted topological phases agree well with available angle-resolved photoemission spectroscopy (ARPES) measurements, in particular the topological phase changes between TlBiSe\(_2\) and TlBiS\(_2\). Moreover, we propose that Weyl semimetal can be realized at the topological critical point in TlBi(S\(_{1-x}\)Se\(_x\))\(_2\) and TlBi(S\(_{1-x}\)Te\(_x\))\(_2\) alloys by breaking the inversion symmetry in the layer by layer growth in the order of Tl-Se(Te)-Bi-S, yielding six Dirac cones centered along the \(\Gamma-L\) directions in the bulk band structure.