a1,a2,a3为整数,
∴1=a1+a2+a3>=3(a1a2a3)^(1/3),
∴a1a2a3<=(1/3)^3=1/27,
∴1/a1^2+1/a2^2+1/a3^2>=3[1/(a1a2a3)^2]^(1/3)>=3(27^2)^(1/3)=27.