assuming you are getting the Yonex racket. 3U : weight of the racket G5 : size of the handle do some searches, you will find more details.....
are u sure? i tought 3UG4 was the most commonly used weight/size , cuz look at all the low end and mid end rackets, their almost all G4, u walk into a store and G4 is the most common grip size anyways, idk about weight depends, all low end rackets are like around 2U, then once u get into mid and high end its all about personal perference i guess, anyways, i will be nice and post the specs but next time do a search please: weight: 2U 90-95g 3U 85-89g 4U 80-84g now not all companies work this way, i belive those are the exact yonex specs, i didnt check so what eva i know that some companies work the other way and 4U is the heavyest and 2U is the lighest, their is also some companies that make a 5U rediculusly light haha so what out make sure you check the weight.
