‘Scarcely’ and ‘Hardly’ are followed by ‘WHEN’ and not by ‘THAN’.
Ex.- I had Scarcely entered the room WHEN the phone rang.
Hardly had he seen his father when he stopped smoking.
Scarcely had he entered the room when the light came.
Hardly had he reached the school when it began to rain.

‘Though’ is followed by ‘yet’ and not by ‘but’.
Ex.- Though he is poor but he is honest (Incorrect)
Though he is poor, yet he is honest. (Correct)