For similar figures, the ratio of areas is the square of the ratio of linear dimensions (such as radius).
We assume that by "25 times greater than" you mean "25 times". If that is the case, the radius of circle B is 3*√25 = 15 units.
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If you really mean the area of circle B is 26 times that of circle A (25 times greater), then the radius of circle B is 3*√26 ≈ 15.297 units.