That's how much he's grown in the season. He started as an 81 and has grown to an 84, thus the +3 appears in the squad report.
Another example.
Hugo Lloris starts out as an 86 I do believe. So when I go to squad report, his overall is now a 90 with a +4 by it. When I go into squad management, his overall is a 90. That's how the system works.