Start from the top.

3 guys, 10 pounds each. 30 pounds.

Shopkeeper makes a mistake, the total is 25 pounds.

Mates decide that it is a decent thing and give the shopkeeper and extra two pounds. Total for the TV is now 27 pounds.

Each of the mates pockets a pound each. For a total of 3 pounds change.

The confusion is that the Cost of the TV is 27 pounds and the guys have 3 pounds change to add to that total (not the amount the shopkeeper pockets for honesty).