Each of these tabs can be used to write code using the corresponding language.
All 3 of the previous mentioned languages can even be used within the same file, like this:
<h1> This is a h1 header </h1>
<p> This is a paragraph </p>
However, as best practices dictate, it is advisable to separate the design in 3 different files; one for each of the 3 different languages. This is the reason why the widget's code editor has a dedicated tab for each language.