These last three episodes are connected to the main anime in the sense that they give more detail about Rimuru as a teacher but there's also other connections with the main story making these episodes worth watching if you're interested in the story of the anime.
The characters are all the same as in the anime except for extra teachers and minor characters. The animation, voice acting and sound are all the same as in the anime.
Overall the first two episodes are worth watching if you like fanservice but you can just skip them if you don't. The last three episodes have more of a connection to the story making them worth watching for anyone who's interested in the story or just likes the anime.
