Addition of two number using array of unequal size
Define an API

The first thing you should do is define a function to wrap the implementation.  This gives it a name and a defined interface.  You can then set expectations such as the first number is the largest, the caller needs to pre-allocate a buffer for the result, etc.

Naming

It would be easier to follow your code if i and j had more meaningful names.  num3 could be target_number or sum etc.

Unused variables

You define variables z and digi, but don't use them.  If you don't need them, get rid of them, they simply add unnecessary noise.

carry?

Do you really need your carry flag?  You could simply combine the code that sets it with the code that checks it:

if(num3[i+1]&gt;9)
{
    num3[i+1]=num3[i+1]%10;
    num3[i]=1;
}


Bugs

Your code has two major issues.


As pointed out by @chux in the comments, if num1 has the same number or more digits than num2 your code skips the addition step and then accesses num3 to print it out.  This will probably crash because num3 hasn't been allocated.
If num1 has more digits than num2, you access a negative index of num2.  Once you pass the left side of the shortest number you should stop adding from it.