Task Restriction in Projects

 Hi,

In a project I have 3 tasks say,

1. Drawing Preparation

2. Drawing Approval

3. Purchasing

Task 2 is dependent on task 1 and can not be started until and unless task 1 is completed. Similarly task 3 is dependent on task 2 and can not be started until and unless task 2 is completed. This is what my requirement is.

Now I want to know  -- Say my drawing task is in progress and still not completed. But still can I use task 3 in purchasing module(by giving project and task 3 in po distribution line)for purchasing any material for the project? Or system will restrict me from doing so since task 3 is dependent on 2 and 2 is dependent on 1 and task 1 is not yet completed?