Both "task" and "job" can refer to a piece of work.
- My first task/job will be to get to know my way around the office.
But "task" is often a hard or unpleasant duty.
- You did a good job! You do good work!
"Job" is a countable noun.
- You get a job to earn a living.
"Work" is an uncountable noun. It refers to the activity, nature or place of a job or business.
- The work on the new airport should be underway soon.
- What kind of work do you do?
- You can reach him at the office. He should be at work by now.
- Learning English can be hard work.