A player's +/- stats do not always appear in the box score, but I'm sure that the staff keeps track of them and puts a huge emphasis on how a player helps or hurts the team when he is on the floor. Last night , for example, Jackson has a +3 in his 28 minutes of playing time. If everyone else matched this number, we win going away. Like it or not, the team played better when he was on the floor.
In regards to Lewis, a few games ago he scored something like 10 points but had a -7 in terms of +/- when he was on the floor. He might have been putting up some points, but we were losing the game while he was doing so.
The +3 was likely because when he was in they weren't playing with Jacobson and Young at the same time. Didn't have really anything to do with Jackson. Plus minus for an individual without context doesn't provide you much. But if you want to look at plus minus, sports reference has a box score plus minus and Jackson is the worst on the team out of the players that get meaningful minutes. By far the worst at -1.2.