It might be best if you separated out the different options into their own summary fields.
[# of Workflow Steps]
[# of Workflow Steps - Not Started]
[# of Workflow Steps - In Progress]
[# of Workflow Steps - Completed]
Then your formula logic will be easier to manage:
If(
[# of Workflow Steps]=[# of Workflow Steps - Not Started], "Not Started",
[# of Workflow Steps]=[# of Workflow Steps - In Progress], "In Progress",
[# of Workflow Steps]=[# of Workflow Steps - Completed], "Completed",
[# of Workflow Steps - In Progress]>0, "In Progress",
"Not Started")
