Well... I'd start by matching the names up...
Change:
A X1,X2,X3;
A1.setval();
A2.setval();
A3.setval();
int a;
cout<<"Enter Account No: ";
cin>>a;
if (a== A1.acountNo)
To:
A A1,A2,A3;
A1.setval();
A2.setval();
A3.setval();
int a;
cout<<"Enter Account No: ";
cin>>a;
if (a== A1.acountNo)
And see if that fixes it...