See this WIP documentation that was just published: source/rtd_pages/insight_explorer.rst · 954ac2cf4373cae408efd0838d8b468ea6018caa · zcash-docs / zcash-docs · GitLab
As noted on other thread some Insight explorer changes are being incorporated into the main zcashd so once they are merged these RPC methods will be available if enabled. You can actually use these already though assuming you are running the Docker Insight container as that patched version of zcashd contains them.
Using your example above for the address balance. First SSH into the container:
docker exec -it zcash-explorer /bin/bash
Then use the getaddressbalance RPC method:
/home/zcashd/zcash-patched-for-explorer/src/zcash-cli getaddressbalance '{"addresses": ["t3gQDEavk5VzAAHK8TrQu2BWDLxEiF1unBm"]}'
{
"balance": 1475000000000,
"received": 1475000000000
}