On diagram, 192.168.0.1 is a LD-417 content switch with round-robin balancing

192.168.0.2 and 192.168.0.4 are running squid and doing NAT.
Squid is running fine via 192.168.0.1
When i set my default gateway to either 192.168.0.2 or 192.168.0.4 - all works perfect.
But when i set 192.168.0.1 - packets are not reaching any of balanced servers.
Is there any solution for that?
Thank you.