Something like
minimize the sum of the distance from p to p1 and the distance from p to p2 and the distance from p to p3 with p1 = (1,0) and p2 = (0,2) and p3 = (-1,0) and p = (x,y)
should do. But in my WolframAlpha Android App this query is not understood.
**Postscript: **
Sorry. This is not what you need. It will give the center of gravity, i.e. mean, of the three sensor locations. Anyway, you'd better handle that problem directly within Wolfram Language, not WolframAlpha. The latter's natural language queries are a nightmare concerning mathematics. Most of the time they are not understood and it's and endless trial and error game to achieve results.