With the story/plot of Rambo captured at the Vietnamese prison camp.
Then Rambo 3 could've been what Rambo 2 became to be which at the end it results to Rambo's story/war coming to full closure and you have the "perfect" trilogy.
Also, they didn't really do prequels in 1985. Hell, they were just figuring out sequels and a lot of actors were reticent to do them. Also, while First Blood was successful, it didn't establish Rambo as a brand the way the sequel did. At the time, a prequel wouldn't have made sense.
Also, Stallone looked far different than he did when First Blood was made. It would have been a tough sell to convince audiences he was younger than he was in 1981.
This is the problem with movie audiences today....they don't want any mystery or imagination to their protagonists, they want to see everything that has ever happened to them as a narrative from childhood. It's better that we imagine Rambo's exploits during Vietnam and his escape. They could never do it justice.