3G simply refers to the general name for third-generation technologies for mobile phones. 3G was created since the older 2G version cannot provide a strong internet connection for some phones and various internet activities on those phones. 3G comes with an improved internet connection that can be used for features like video call, sending of voice note and other things that require strong internet connection.
HSDPA, on the other hand, is the acronym for High-Speed Downlink Packet Access). As the name implies, it is used together with 3G to offer a stronger and faster internet connection. Since a lot of people are now using 3G, HSDPA was created to improve the internet connection by making it more faster. While 3G service offers additional features, HSDPA is only meant to develop and enhance the internet connection of 3G. However, most phones are designed to support 3G, while just a few uses HSDPA.
3G stands for the third generation of technologies used in mobile phones. The multiple technologies all provide the same service. HSDPA stands for High-Speed Downlink Packet Access. It is an addition to 3G technologies and it provides faster speeds. Most mobile phones use 3G support, but there are some that don't offer any HSDPA support.
With the introduction of 3G came features such as video calling and TV streaming. With these features, HSDPA provides better latency (time between request sent and to get a reply) compared to 3G. HSDPA is also easier to use and cheaper in price than 3G.