If all these workers have the same level of skill, why does worker 3 have a LARGER marginal product than worker 2

According to the law of diminishing marginal returns, labor productivity will first increase as the total number of workers increase (due to synergy), but at some point, as more units of labor (or other factor) are used, their productivity will start to decrease and each unit added will yield a lower marginal return.

When you look at how teams work, when the first members start to work together, the outcome will be higher than if they worked individually. But if the team keeps growing, and the other factors of production remain the same, e.g. the number of computers remains at 3 but there are 4 team members, then the total output might increase but at a lower proportion (productivity decreased).