I think he's asking whether it should be a single box, containing all three stages, like Dustbowl (look in hammer and you can see), or 3 boxes that are unconnected but in the same vmf, like I'm fairly certain snowplow is.
The answer is a single box in almost all scenarios. Each stage should look like a progression from the previous, so you'll want out of bounds features from the previous stage visible. Basically, this will give the impression that the 3 stages are happening in the same area. Snowplow did it differently because the stages are actually in different areas - a snow clime and an alpine clime.