Reduction from $L$ to $L_{TM}$
If $L$ were decidable, then you could make a decider for $A_{TM}=\{\langle M, w\rangle\mid M\text{ accepts }w\}$ by deciding $\langle M, M, w\rangle$. We know, however that that's impossible, since $A_{TM}$ is undecidable.