设此三数分别为：a-d,a,a+d,则
(a-d)+a+(a+d)=24,
(a-d+3)(a+d+21)=(a+8)^2
a=8,
(11-d)(29+d)=16^2
d^2+18d-63=0
(d-3)(d+21)=0
d=3,或d=-21,
等差数列为：5,8,11；或,29,8,-13.