A deleted file usually is moved to the Mac Trash for temporary keeping. To thoroughly delete the files from your Mac, you need to delete them again from the Mac Trash. If deleting files from Mac Trash takes too long, it could be annoying. Read this post to know how to quickly remove files from Mac.
Although the deleted Time Machine backup is 1.2GB, a large item, it shouldn't take 3 days to delete it from Mac Trash. And the deleted Time Machine backup should be moved to the Trash folder in the external hard drive instead of the system Trash folder. If Mac doesn't have enough space for such as 1.2GB file, it may take too long to empty the Mac Trash. Then, freeing up Mac hard drive space may work to fix this issue.
Other reasons for the Trash slow to empty issue could be the item you put in Mac Trash is still being used, being locked, the item is a system file, malware infection, etc. For your case, you can try the following solutions:
1. Restart Mac and then try to clear the Time Machine backup from Mac Trash again.
2. Remove files from Time Machine with Terminal: type command sudo rm -R and drag-drop the unwanted folder from Mac Trash to Terminal, then hit Enter to run the command.
3. Use a third-party cleaner for Mac such as iBoysoft DiskGeeker. It can check junk files and unwanted files from both Mac hard drive and external hard drive. Within a few steps, it can quickly and permanently delete selected files from your Mac and connected storage media.