You can't directly convert liters (a unit of volume) to millimeters (a unit of length).

Here's why:

* Liters (L) measure the amount of space a substance occupies (volume).

* Millimeters (mm) measure the distance between two points (length).

To make a meaningful conversion, you need to know the shape of the container holding the 3 liters.

For example:

* If the container is a cube: You could calculate the length of one side of the cube in millimeters.

* If the container is a cylinder: You could calculate the diameter and height in millimeters.

Let me know if you have more information about the shape of the container, and I can help you calculate the dimensions in millimeters.