Your code never prints any spaces, that should be a problem.
You can use this simple approach:
for (int i = 0; i < 3; i++) System.out.println(" @@@".substring(i, i+3));
The logic is quite simple: you have the string with two spaces and three at-signs. The first line of output needs to be two spaces and a single at-sign, so that's the first three chars of the string. The second line should be one space and two at-signs—that's the three chars of the string after skipping the first one; and so on: you just slide through the string, each time skipping one more from the beginning and taking the next three chars.
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