This is a straight-forward problem by Geoffrey Mott-Smith from 1954.

“Three tangent circles of equal radius r are drawn, all centers being on the line OE. From O, the outer intersection of this axis with the left-hand circle, line OD is drawn tangent to the right-hand circle. What is the length, in terms of r, of AB, the segment of this tangent which forms a chord in the middle circle?”

Answer.

See An Intercept Problem for solutions.