These three terms look the same, and beginners usually get confused. However, all these terms are completely different.
Trimming means removing some frames of the video from the ends. You can trim a video from its start or end. When you trim the video, the selected frames are deleted, and the rest of the video remains unaffected.
Cropping means reducing the size of the frame. Every video has a certain size of the frame, or it is also called resolution. The frame size could be 1920 x 1080, 1920 x 720, or anything else. When you crop the video, some part of the frame is deleted. It means you cut the excess part of the frame. You can select the part of the frame that you want to retain, and the rest will be deleted. Keep in mind that it does not affect the size of the objects in the frame; everything looks the same.
Cutting is usually referred to as splitting a video. When you want to cut the video into two or more parts or if you want to remove the middle of the video, you need to cut it.