They mean pretty much the same thing. I think what would be most helpful is to know which ones are more applicable to certain subjects, and which are used more often. They all mean "one part of a whole" but: When talking about food, we use "portion" very often. Example: I gave each child a portion of the food. Example: I try to only eat a small portion of dessert. Section is the most versatile in terms of use. You can talk about a section of a building (like a room or area). Or a section of a website. Or a section of some cloth or other material you're cutting into separate parts. Or a specific part of a book. Example: Here is the science fiction section of the library. Example: In this section of the textbook, it talks about verb tenses. Example: The wood was measured and cut into three sections. "Segment" is used a lot less often than "section". If you're in a mathematics class and discussing lines, you'll hear "segment" a lot. But otherwise, "section" is usually a better choice. Example: This segment of the line is on a separate plane.

I would have to give you case usage as in some cases they are interchangeable. Here are where they are not interchangeable. I think there is reference to the whole in segment. (I cant justify this properly its just a feeling) haha. We say orange (果）segment. We can't use the other two. Section （i think the cutting root of this word is more relevant) haha Probably closer to part. We can say a VIP section or history section. Again we cannot use the other two here. Portion (use as a lumping or grouping) haha We can use this for food generally. Like one helping. Or a portion of fish and chips. We cannot use the other two here.