do I need to “know” several openings?


check it out...
https://www.youtube.com/results?search_query=chess+opening+repertoire
Chess Openings Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell
@1
"trying to be prepared for them" ++ You cannot prepare for everything.
"Is it just part of the learning process?" ++ The learning process is more about tactics and endgames. You will not lose or win bbecause of the opening, but because of tactical mistakes.
"are there specific things I can focus on to generally improve my game?" ++ Analyse your lost games.
"300-400ish rating" ++ That is a sign of frequent blunders. Always check your intended move is no blunder before you play it.


A blunder is a specific type of mistake which typically risks the IMMEDIATE net loss of material or worse - the threat of imminent checkmate of the blunderer. Whereas an inaccuracy, while it technically could be considered a mistake, is not immediately harmful and typically does not risk the loss of material or checkmate, at least not in the short term. So it's blunders that you must absolutely try to avoid. The best way to train yourself to avoid making blunders is, as a previous commenter said, is to do a blunder check of your completed games. The chess.com post-game analysis function will identify and indicate the blunders, inaccuracies and any other mistakes in your completed games.

Don't try to brute-force learn all openings; there's about a thousand (!) of them.
Just pick a few you like and go in depth until you see the underlying connections toward checkmate. (this takes years so be patient with your progress)
Once you see chess as a whole, you can diverge and include more opening systems into your play.

Atleast 3 openings, 1 as white which should be your pet variation, 2 as black 1 defence against e4 and 1 against d4

for openings, instead of memorizing all of them, pick a few to memorize and for unfamiliar ones, just improvise something that follows opening principles.

You do not need to "know" several openings. If you look at my games, you will find that my opponent and I are "out of book" in a handful of moves. Yet, I am in the top 5% of Rapid and Daily players.
I don't study openings, but I do go over my games, and compare my opening moves with opening moves in Master games. Eventually, I make "decent" moves by intuition rather than relying on my memory.
Note: If I wanted to be a top player, I would have to study openings, but Chess is just a hobby with me.

No just master a few sound opening. As you go higher in rating then learn how to refute some unsound gambit.
Later on, you may just need to have some knowledge on some of the most popular openings.
There are some openings with black that depend on the opening by white. Stay away from them. Only titled players really need to know multiple openings mostly because of the traps.
If you’re struggling to get playable middlegame positions then I’d recommend really learning and focusing on opening principles and following them religiously. If you are still struggling then I’d learn a few common opening theories. Our games are won or lost because of blunders at our level.
-Jordan