Final answer:
The 3rd largest city on the Red River in question is Shreveport, located in the state of Louisiana.
Step-by-step explanation:
The question you're asking is about the state's 3rd largest city located on the Red River, which is Shreveport. Shreveport is in the state of Louisiana. This information pertains to geography, as it concerns the location and ranking of cities within a state. The fact mentioned about a billboard with a religious message is not directly relevant to identifying the state but gives cultural context to the city of Shreveport.