My guess is someone bought the 3rd panel and forgot to buy the connectors they needed, than never got around to it and sold the RV.
first what controller do you have? If its 20A add the 3rd panel into the system.
using T
his tool, you can take those connectors apart, than just install the additional panel into your system by hooking positive to positive, negative to negative (parallel connection) using
adapters like there.