I don't know how you managed to get $0.25$, but the reason your calculator says that it's $4$ is because that actually is the right answer.
An equation generally means what you have on one side of the equation is equal (that's what the equals sign does) to that which you have on the other side of the equation. In other words, the number on the left should be exactly the same as the number on the right. This means that $12$ is the same number as $3x$. This in turn means that dividing $12$ by $3$ numerically should be equivalent to dividing $3x$ by $3$. Dividing $12$ by $3$ gives you $4$ and dividing $3x$ by $3$ gives you $x$. So, the result that you get is:
The order in which you read an equation does not matter. It's the same thing either way you look at. So, the result above states that $x=4$. And that's your final answer.
You can check your work by replacing $x$ with $4$:
As you can see both sides contain the same number, as it should be.