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Can I say,

(a) The three / Three boys are cycling.

(b) My hobby is playing my guitar.

(c) The bank was very crowded.

(d) My parents were scolding me because my exam is very poor.

My parents were scolding me for my poor result in the exam.

(e) They were walking although the day was hot.

(f) They are excited for watching the television now.

He watched the exciting programme from the television.

(g) He jumped into the sea for saving the drowning boy.

(h) He folds his blanket after getting / waking up.
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Use these, Vincent:

(a) The three / Three boys are cycling.

(b) My hobby is playing my guitar.

(c) The bank was very crowded.

(d) My parents were scolding me because my exam results were/are very poor. -- AmE at least prefers the plural

My parents were scolding me for my poor results in the exam.

(e) They were walking, although the day was hot.

(f) They are excited (by) watching the television now.

He watched the exciting programme on the television.

(g) He jumped into the sea to save the drowning boy.

(h) He folds his blanket after getting / waking up.
Comments  
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(f) They are excited (by) watching the television now. (should I put "the"?)
If you are talking about the medium in general, don't use 'the'.
If you are talking about a specific TV set, use 'the'.