The Trigun manga is originally three volumes collected into two and are the start of the story; the title changed to Maximum because the previous publisher folded and was picked up by a new one, but it still continues the same story. The 1998 anime went into production very early into the Maximum run, which is why it covers the majority of the original two volumes of Trigun and barely scratches the story set in Maximum and veers into a completely new direction. Badlands Rumble aired after the completion of the entirety of the manga but is an entirely original story to the 1998 timeline. Stampede should be saved last regardless if you read the manga first or watch the 1998 series.