So I know that the universe is flat in a 4D point of view, but what about a 3D point of view?
You seem to be misunderstanding the term "flatness". In this context, it does not mean the universe looks like a piece of paper in any sense. It instead refers to whether space is warped.
i'm just a kid who is really interested in the universe
I'll try to simplify as much as possible. I'm assuming you've learned that triangles have 180°. For the mathematics you'll be dealing with in middle school, high school and even most of college, you'll only have to deal with that.
The more accurate statement is that triangles have 180° when our geometry is not warped. When triangles have 180°, we say we are dealing with Euclidean geometry (named after Euclid, an ancient Greek mathematician). There are in fact cases in higher mathematics when this does not hold, which we call Non-Euclidean geometries. Just as a side note: Euclidean geometry was developed thousands of years ago. On the other hand, Non-Euclidean geometries only began to develop in the 1800s, so they're pretty recent.
Anyway, Euclidean geometry deals with flat spaces. For example, if you draw a triangle on a piece of paper, it will indeed have 180°. However, Non-Euclidean geometry deals with curved (warped) spaces. For example, if you draw a triangle on the surface of a ball (which is not flat), the triangle will actually have more than 180°. If you instead draw the triangle on a potato chip or saddle-shaped surface (also not flat), it will have less than 180°.
So with regards to the Universe: let's say we draw a triangle connecting three stars in outer space (we can actually use any object or simply points, but I'm using stars as an example). Will that triangle be less than, equal to or more than 180°?
According to our measurements, it will actually be equal to 180°; the Universe is indeed flat. This has major implications for physics and astronomy.
i'm sorry (in advance) if this is a simply answered bad question.
Future advice: don't be afraid to ask questions. Yours wasn't bad and is pretty common.