He has been studying _____ three hours."

• "for" is used to denote a duration of time (e.g., "for three hours").
• "since" indicates a starting point in time (e.g., "since 9 a.m.").
• "from" generally denotes the starting point of a range (e.g., "from Monday to Friday").
• "during" indicates within a particular period (e.g., "during the meeting").

Example:
- He has been studying for three hours. (নির্দিস্ট সময় ধরে)
- He has been studying since 9 a.m.. (একটা নির্দিস্ট সময় থেকে)
- He has been studying from 9 a.m. to 12 p.m.. 
- He has been studying during the meeting.