I don't think he got an impact time.
The first signal that triggers is then defined as t=0. The others are delta values (so called TDOA) to the first dtn=tn - t etc. So, when you put up your equation of the circles, your radii of 3 circles will have an unknown delta in common for which you have also to resolve (resolve for x,y, and r). BTW, this problem is actually called multilateration Trilateration - Wikipedia.