Is 350 ELO after 3 days alright?

Ratings are just a reflection of where you are in your journey. Right now, the good news is you have many opportunities to improve. Honestly that's kind of a cool place to be because you have lots of things to learn and when you're able to implement them, you can look back and see how far you've come.
If you want to improve your middle game, start by studying endgame first. Knowing which positions offer you wins, converting winning positions, and avoiding losing positions act as a compass that points true north for your path through endgame.
Make sure your openings allow solid development and allow you to have all of your minor pieces in active positions where they can coordinate against your opponents' weaknesses. Once you're out of the opening, look for hanging pieces, weak pawns and poorly defended kings. You'll make mistakes but that's part of the process.
Hope this helps.

Improving Your Chess - Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell