Enables 5'-(N(7)-methyl 5'-triphosphoguanosine)-[mRNA] diphosphatase activity and 5'-(N(7)-methylguanosine 5'-triphospho)-[mRNA] hydrolase activity. Involved in RNA decapping. Is active in glutamatergic synapse. Orthologous to human NUDT3 (nudix hydrolase 3).