肥 and 胖 are usually used to describe body weight. For example, “胖子”, “肥胖”, “肥肉”. The difference is that compared with 胖, 肥 is much more offensive. 博 has a positive meaning. We usually use this word to describe culture and knowledge, something abstract. You can search “博大精深” and “博学” online and see their meaning. It may help you understand 博 deeper.

No difference in meaning. 肥 is more formal. 博 means profound

I don't know write the博's mean，maybe a write wrong？But the 胖 means someone have many meat and burliness。while the 肥means some one is fat and have much grease 。For example，I often told to my mom，I don't like to eat 肥meat.（attention，i use 肥 not 胖）。in my opinion，肥 than 胖 more let people dislike。whatever，if say somebody 肥 or 胖 are all very hurt that people 。Can you make sense of what I mean? My English is poor but I still hope to help you with your Chinese. If I have any mistakes welcome to point them out。Thanks～