So that each row and column of 3 digits sum to the same number. I was able to find a number of solutions. The interesting thing was that some of the solutions were related to each other. Besides the rotational symmetry, there is the property that given one solution, if you replace each number $a$ by $9-a$, you get another solution. There seems to be a more subtle relationship between some solutions but I haven't yet nailed it down. I don't want to spoil, so I have to be vague. It looks like if a column is $a, b, c$ in a solution, then there is another solution with $a, c, b$, and the other 3 rows permuting similarly.
My question here is really to ask for references to this puzzle. I know I just ran across it in the last month or so, but I can't find it on Google or StackExchange or anywhere. I'm curious as to whether one solution leads to all the others by rotation, by the $9-a$ rule or by some permutation trick.
If anyone cares, I can share some of my solutions and the reasoning that led to them.
EDIT: As I said, I have solutions, so I'm not asking for solutions, but relationships between the solutions. The $9-a$ trick is an example. I think there is some permutation trick that will turn some solutions into others, as I mention in the comment below.