I tried couple of things that you can see here: https://scripter.co/notes/plantuml/#simple-flowchart
But I was unable to draw a simple flowchart with these 2 rules:
The flow chart should go from left to right
No start or stop markers
(*) -> "First Activity" -> "Second Activity" -> "Third Activity"
"First Activity" -> "Second Activity" -> "Third Activity"
:First Activity; :Second Activity; :Third Activity;