Answer:
Third parties often focus on one, particular issue.
Step-by-step explanation:
The first option is not correct because Third parties don't have more official members than the two main parties.
The second option is not correct because usually, these parties are newer on the political scene.
The third option is correct because Third parties very often use a strategy of focusing on one particular thing, something that is influencing big portion of the population and there is revolt about the managing of that by the two main parties.
The fourth option is not correct because in most casses the two main parties are the oldest on the political scene.