When I did my undergraduate thesis, my adviser was expected big on students expecting them working really hard, so it ended up as a reduced master's thesis (most of the reviewers said that it was an overkill for a B.E).
So because of this, I did my master's thesis pretty much in the same way I did my bachelor's thesis; the PhD dissertation was a different story.
What are the principal differences between these three pieces of research?
In a Bachelor or Master thesis, you have to show that you are able to apply the knowledge of your field to solve a typical problem in your field.
In a PhD thesis, you have to show that you are able to extend the knowledge of your field to solve new problems.
The distinction between a Bachelor and a Master thesis may be a bit subtle. Generally I think a Master thesis should show a significantly greater level of independent working. While for a Bachelor thesis your advisor could tell you which methods to use to solve a specific problem, for a Master thesis you could be expected to choose the methods on your own, and maybe adjust existing methods to better fit the problem at hand.
These are the principle differences I see, but of course its also depending on the specific student. A good Bachelor thesis can be better and include more independent work than a bad Master thesis.
I would see the differences as follows:
Undergraduate thesis demonstrates the capacity to apply basic research skills in an area of interest to you. At this level, the focus is on gaining broad competencies (akin to an overview of what research is all about).
Masters thesis demonstrates the capacity to apply advanced research skills (i.e. move beyond basic research skills) in an area of interest to you so that you are able to incorporate some critical insights in your study. At this level, the focus is on developing critical thinking in a subject area.
PhD thesis demonstrates the capacity to apply specialised research skills (i.e. expert knowledge of a particular concept or method) in an area of interest to you so that you can make significant and original contribution to knowledge. At this level, the focus is on identifying a 'gap' in knowledge and addressing it, hence you advance knowledge in a field of study.
These are are arbitrary descriptions as there are overlaps (i.e. all levels require critical thinking); however, the three levels are distinct and require difference level of competencies.
