The words "goal" and "objective" are usually used to describe stages or intermediate steps in a larger project.
For example, a person might say, "His goal was to be the best in the class that year". This was a step in his education.
A person might say, "The first objective was to capture the fort". This was a step in the overall conflict.
The word "purpose" is used more accurately to describe a more final and fixed determination.
For example, a person would say, "My purpose in starting this retail business was to offer people quality goods at lower prices". This would be an ongoing endeavour.
Hope this helps