3-point steiner tree

Hello,

It is well-known that the exact solution of the Euclidean Steiner tree
problem with 3 points
or terminals can be found in polynomial time. I would like to know
whether there exists a polynomial algorithm to find the exact solution
for the steiner tree problem on a weighted graph.
That is given a weighted graph G(V,E,w) and a vertices subset
S\subseteq V (|S|=3) find a tree of minimal weight which includes all
vertices in S. Thank you.


Best,
Chris

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