Here's the `user.py` file for the specified routes:

```python
from typing import List
from fastapi import APIRouter, Depends, HTTPException
from sqlalchemy.orm import Session

from app.api.v1.models.user import User
from app.api.v1.schemas.user import UserCreate, UserResponse
from app.api.core.dependencies.dependencies import get_db

router = APIRouter()

@router.get("/users", response_model=List[UserResponse])
def read_users(db: Session = Depends(get_db)):
    users = db.query(User).all()
    return users

@router.post("/users", response_model=UserResponse)
def create_user(user: UserCreate, db: Session = Depends(get_db)):
    db_user = User(**user.dict())
    db.add(db_user)
    db.commit()
    db.refresh(db_user)
    return db_user

@router.get("/users/{user_id}", response_model=UserResponse)
def read_user(user_id: int, db: Session = Depends(get_db)):
    db_user = db.query(User).filter(User.id == user_id).first()
    if not db_user:
        raise HTTPException(status_code=404, detail="User not found")
    return db_user
```

Explanation:

1. We import the necessary modules and dependencies.
2. We create an instance of `APIRouter` to define the routes.
3. The `read_users` function handles the `GET /users` route. It fetches all users from the database using `db.query(User).all()` and returns them as a list of `UserResponse` models.
4. The `create_user` function handles the `POST /users` route. It takes a `UserCreate` model as input, creates a new `User` instance from the input data, adds it to the database session, commits the changes, and returns the newly created user as a `UserResponse` model.
5. The `read_user` function handles the `GET /users/{user_id}` route. It fetches a user from the database based on the provided `user_id`. If the user is not found, it raises an `HTTPException` with a 404 status code. Otherwise, it returns the user as a `UserResponse` model.
6. All routes use the `get_db` dependency to obtain a database session.

Note: This code assumes that you have defined the `User` model in `app.api.v1.models.user` and the `UserCreate` and `UserResponse` schemas in `app.api.v1.schemas.user`. Additionally, the `get_db` dependency should be defined in `app.api.core.dependencies.dependencies`.