It would help if you pasted your output, and also what you expected to see.

I suspect you wanted to see something like:

hello [ps output]
world
hello [ps output]
world
hello [ps output]
world
[download]

But instead you are seeing something like:

hello hello hello [ps output]
[ps output]
[ps output]world
world
world
[download]

You answered your own question- the 3 forks you did are running async.