Let the diameters of concentric circles be k, 2k and 3k.
∴ Radius of concentric are k2,k and 3k2.
∴ Area of inner circle A1=π(k2)2=k2π4
∴ Area of middle region, A2=π(k)2−k2π4=3k2π4
[∵ Area of ring =π(R2−r2). Where R is radius of outer ring and r is radius of inner ring]
And area of outer region A3=π(3k2)2−πk2
=9πk24−πk2=5πk24
∴ Required ratio =A1:A2:A3
=k2π4:3k2π4:5πk24=1:3:5