In this paper a study to application of Least Squares Finite Element Method (LSFEM) is made and with auxiliary equations (temperature derivatives) in the solution of Transient Three-dimensional Diffusion-Reaction. In order to do so, two applications are presented and discussed, one of them Pure Diffusion and another DiffusionReaction, both solved towards the constructive meshes with hexahedron of 8 and 27 nodes. This analysis uses the standard L∞ (maximum error in all meshes) and L2 (average error in all the meshes) to verify the numerical error committed in the solution9196209CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP500382/2011-52014/06679-
