How to implement the API Gateway pattern

Networking
1

Hey folks :wave:
I have an api made of a few services (s1,s2,s3) sitting behind istio-ingressgateway:

                -- s1
               /
ingress --- vsvc -- s2
               \
                -- s3

Now I want to introduce a consumer of that api, logically looks something like this:

                  -- s1
                 /
ingress ----- ? ---- s2
      \       |  \
       \     /    -- s3
        webapp

How do I implement this?