You ask a good question and, while I appreciate the concept of "to a middle extent", the answer to your predicament is actually more straightforward.

To understand how the word "extent" is being used, let's first define it. The Oxford English dictionary has as the pertinent definition, "the amount to which something is or is believed to be the case." From this definition, we can see that you could also substitute the word "degree" for "extent" in this instance. When we ask, "To what extent is XYZ", we are asking, "To what degree".  

If we are talking about just one thing, we would say to a "great extent", as you have noted in your question. When we are talking about more than one thing, however, we are making comparisons. So you could say that your first topic is to a "great extent", that your second topic is to a "lesser extent", and your third topic is to "an even lesser extent." (However, "to an even lesser extent" reads awkwardly, so you might just want to compare the second and third topics to the first and say that ideas two and three are both "to a lesser extent".)