Unit impulse response of a cascade interconnection of three discrete-time systems
HINT:

Consider the cascade interconnection of three causal LTI systems, [...]&quot;

What does that imply for the impulse response $h_1[n]$?
What is more, since the total impulse response is causal, and you know from its definition that $h_2[n]$ is causal, $h_1[n]$ must also be causal.