A. 'Across' refers to moving from one side to another.
Example:
1. Mr. Sinha travelled across America on his motorcycle.
2. Gita and Juli are swimming across the lake.

B. Through refers to moving directly inside something and out the other end.
Example:
1. The bullet Ben shot went through the window.
2. The train passes through the tunnel.

c. Into refers to entering or looking inside something.
Example:
1. Shyam went into the room.
2. They stare into the darkness.