There are a few bugs , some cases you missed :)

Bug 1 :

When you did this :

if(ans == 1)
{
for(int i=0;i<n;i++)< br=""> {
if(a[i]%3 == ans)
{
pos = i;
break;
}
}
// cout << ans << " " << a[pos] << endl;
for(int i=n-1;i>=0;i--)
{
if(pos!=i)
cout << a[i] ;
}
}

You've done for the case if there were some digits such that digit %3 == 1

But what happens if there isn't.

Try to think how to solve then ( Hint : You managed to do that for ans==2 )

This bug fails the code for tests like :

5
3 2 2 0 0

actual output : 300
your o/p : 32200

Bug 2 :

Even if you find a digit such that digit%3==1 , you should check are all the remaining digits 0 or not. This will prevent you from printing leading zeroes.

Your code fails this case :

3
0 0 1

actual : 0
your : 00

This may be the case for test case 14 where your code failed

Bug 3 :

Again when you did this :

for(int i=n-1;i>=0;i--)
{
if(i!=pos1 and i!=pos2)
cout << a[i];
}

You should take care about if all the other digits are 0 or not.

Your code fails this test :

4
0 0 1 1

actual : 0
your : 00

This may be the case for test case 14 where your code failed

Bug 4 :

Again here :

for(int i=n-1;i>=0;i--)
{
if(i!=ps)
cout << a[i];
}

You should check whether the other digits are all zero or not.

Your code fails this test :

3
0 0 2

actual : 0
your : 00

This may be the case for test case 14 where your code failed

I hope there are no more bugs .Hope this helps . Don't be disheartened by all these bugs. You'll obviously do better and learn from your mistakes :) Good luck :)