Installing TWRP on a Samsung device, no fastboot (using ADB or other ways) My device (Samsung Galaxy S3 I9300) is not detected by fastboot in Download Mode, but it is detected properly by adb. I want to install TWRP as my device recovery. Is there any way to install it using adb instead of fastboot?

There is a question about the problem of detection of the device by fastboot on this site (which has no working answer yet). But my question is not how to solve that problem. If you like to answer that problem please see this page.
There is no fastboot mode on Samsung. You must use the ODIN mode, which is an alternative to fastboot. On Linux there is a client application called Heimdall which talks to phones via Odin mode. So the answer to your question is ODIN or Heimdall. ODIN is the proprietary Samsung PC application, Heimdall is multiplatform open source alternative.

Alternatively, if you are able to root your phone somehow, you can put TWRP (or any other recovery) on your phone by rewriting the recovery partition via shell but this can be tricky and I wouldn't recommend it unless you know exactly what you are doing or willing to risk bricking your phone. If interested, I can write a generic how-to too but I'd rather leave it to someone with more experience on that - I only did it like 5 times.