Build a small arena in which the player collects three objects and reaches an exit. That gives you movement, collisions, state, feedback and a win condition in one scene. A playable loop this small teaches more about finishing a game than a detailed map with nothing to do. Unity's Roll-a-Ball project is a useful official reference for this style of prototype.
Create a world that is easy to test
Start a 3D project in a supported Unity editor, save the scene and create a floor from a scaled cube. Add a sphere above it as the player, with a Rigidbody and a compatible Collider. Position the camera so the floor edges are visible. Run the scene before adding controls. If the sphere falls through the floor, fix the collider or position now. Art, scoring and menus cannot compensate for broken movement.
Connect input, movement and pickups
Using the Input System, define a two-axis Move action and bind directional keys or a stick. Read the vector into a movement component and apply force to the Rigidbody during physics updates. Create a pickup with a trigger Collider; when the player enters, count it once and remove or deactivate it. Make the pickup a prefab and place three instances apart. Tag or otherwise identify the player so unrelated physics contacts cannot increment the counter.
Finish with a testable win condition
Display the collected count. Before all three pickups are collected, entering the exit should explain what remains. After the third, entering the exit should display a win state exactly once. Add a restart action that resets the score and scene. Include the scene in the chosen build profile, produce a desktop build and launch it outside the editor. Check input focus and UI scaling in a different window size before spending time on better models.
For the documented workflow, see Unity pickup tutorial.
Common Questions
Should I start with imported models?
Primitive shapes make physics problems easier to see.
Why do pickups block the player?
Check Is Trigger and the collider setup.
Why does one pickup award twice?
Mark it collected before further events can count it.
When is the prototype finished?
Another player can start, win, restart and understand the goal without instructions from you.