I can't speak for other countries, but in the US workflow is common as well, though different from workload (I have never heard the term workstream personally). Workflow would be the way you go about doing your job, like a list of procedures to accomplish a task. Literally, the flow of your work.

I guess it depends on which country and what your job is. In US you are most likely to hear "workload" on regards to how much work you have to do. "He has a heavy/full workload."

In Britain and Australia too, we say workflow to describe the process of your work. Workload is really all the jobs you have to do. Workstream I've never heard of and spell check here has underlined it in red