Superheroes are fictional characters with 1) ________ powers who use their powers to fight crime and protect the public. Some popular superheroes include Superman, Batman, Spider-Man and Wonder Woman. Superheroes often have backstories that 2) ________ how they have gained their powers, 3) ________ being born with special powers, being bitten by a radioactive spider or being exposed to cosmic radiation. They usually have a specific outfit or clothing that they wear to hide their identity and protect 4) ________ while fighting crime. Superheroes have been a staple of popular culture 5) ________ decades, appearing in comics, movies, television shows, and video games.

Select the most appropriate option to fill in blank no. 3.

The correct option is such as.

Explanation:
Blank number 3 requires a phrase that functions to introduce specific examples illustrating the general statement about how superheroes gain their powers.

Let's evaluate the given options:
1. therefore: A conjunctive adverb meaning "for that reason" or "as a result." It is used to show a consequence, not to introduce examples.
2. subsequently: An adverb meaning "afterward" or "later." It indicates chronological order or sequence of events.
3. instead: An adverb meaning "as an alternative or substitute." It is used when one thing replaces another.
4. such as: A prepositional phrase used specifically to introduce examples of a concept or category mentioned earlier. In this context, it properly introduces the examples of superhero origins ("being born with special powers, being bitten by a radioactive spider or being exposed to cosmic radiation").

Thus, such as is the most appropriate choice to complete the sentence logically and grammatically.