Hi
You must have configured it wrongly....its quite simple..you have to just understand the below topology that you have deployed...
On A-site RV340:
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S2S tunnel1:
(siteA-subnet1)----[RV340-A]wan1-----{internet-isp-router}-------wan1[RV340-B]---(siteB-subnet1)
S2S tunnel2:
(siteA-subnet1)----[RV340-A]wan1-----{internet-isp-router}-------wan1[RV340-C]---(siteC-subnet1)
On SiteC:
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- ensure that the siteC-subnet hosts are configured with the default-gateway ipaddr of RV340-C router's lan-interface (in the siteC-subnet1)
hope this is useful...and helps in solving the issue...