None of your tasks are linked, therefore the task with the longest duration (initially Task a) is the task on the critical path - it is the task that is driving the latest date in the project. When you increase tasks b & C to 3 days duration they now all finish last and are all critical. Changing the start date of A to June 5 with 3 days duration still makes it finish last.

So, what are you expecting?

Julie,

Thanks for your reply. The task with the longest duration is a "critical" task.

I don't know if this is a question or statement. In the project you had attached yes, the longest task was critical because it was the task driving the end date of the project. Critical is calculated mathematically - it is not subjective. You may have a task that you consider to be very important, but it may not be on the critical path.