Clock time and duration
Three o'clock and three hours are different kinds of information. Distinguish 時, 分, and 時間 so that appointment times and durations do not get confused.
Read the hour
The basic readings are 四時 よじ, 七時 しちじ, and 九時 くじ. ななじ is also used for 七時 to avoid confusion. Do not mechanically form よんじ for 四時 or きゅうじ for 九時.
三時半 means half past three, and 午後三時 means three in the afternoon. If morning and afternoon could be confused, clarify 午前 or 午後, or check an explicit 24-hour time.
Sound changes with 分
一分 is いっぷん, 三分 is さんぷん, 六分 is ろっぷん, and 十分 is じゅっぷん or じっぷん. 二分 is にふん and 五分 is ごふん, so the counter is not pronounced ふん after every number.
四時十分 can be read よじじゅっぷん, combining hour and minutes. After reading written times aloud, listen and write them as numbers. Check whether a sound change in 分 led you to hear the wrong number.
Separate time points from duration
三時に始まります。 means that something starts at three. 三時間かかります。 means that it takes three hours. Removing 間 changes the kind of information even though the number remains the same.
三十分 can express duration, as in 三十分待ちます ('I will wait for thirty minutes'). Questions also differ: 何時に asks at what time, while どのくらいかかりますか asks how long something takes.
Check: What do 三時 and 三時間 each mean?
Show the answer and why
三時（さんじ） means three o'clock; 三時間（さんじかん） means three hours. Distinguish an appointment time from a duration.