What can be done if my co-worker and I left 3 beer bottles on the company’s truck but it was after work hours?

You may be questioned about it. Unless you have an employment contract identifying the terms of termination, in CA, employment is considered to be at-will. That means that the employee can quit at any time for no reason, and you can be terminated for no reason as well. So, if the employer wants, they can fire you (unless you have an employment contract). If you have an employee handbook, there may also be a provision on alcohol in there as well (regardless if its during work hours or not).

I assume the company is aware of this and knows that it was you two. Respectfully, they probably can fire you. This is a terminable at will state. What you did was probably not the smartest thing in the world to do.

You can beg for forgiveness. If that doesn't work, you can look for new employment. How does your employer know it was after hours. It doesn't.

I do not see a legal question here. I do not know what you can do, but be more careful next time. The fact that this was after hours would not matter, since you were in a company vehicle.