Explanation
Usage of Beside & Besides.
Beside means adjacent while besides means in addition to/apart from.
There was a small table beside the bed. What do you like besides playing cricket?
Usage of Across
Across has 3 different meanings.
1.From one side to other side. Like He swam across the river.
2.On the opposite/other side. Like we saw some old friends across the road.
3.When something stretches/extends from one side to another. Like there was a bridge across the river.