I can name quite a few, and they tend to be mutually reinforcing. Unequal access to education is one of the most urgent in our society. Income disparity is both a predictor and a function of a lack of education opportunities. Race and ethnicity are, of course, correlated with the previous two situations, and they remain problematic in our society today. In some places, though not as much in the United States, religious minorities suffer inequality, often closely tied to the other factors, as the example of Christian minorities in Sudan or Muslims in the former Yugoslavia in the 1990s demonstrated. Globally, many have argued that economic imperialism contributes to subordinate status, and as some have observed above, that tends to accompany military and technological disparities.
Subordinate status can become very complex. For example, a white person living in a largely white community might seem part of the dominant group, but if that person is (for instance) gay, s/he might face significant discrimination. An atheist might share the same skin color with the the majority of people around him, but he might feel part of a subordinate group simply because atheists are a numerical minority in the U. S. A conservative might be identical in most other respects to her co-workers, but if those co-workers happen to be liberal, the conservative is likely to consider herself a member of a subordinate group, at least in that context. And, of course, the same situation can occur in reverse. In various ways, people can belong to "dominant" groups in one aspects of their lives and to "subordinate" groups in other aspects of their lives.
Most of the previous posts give pretty good examples from history of so-called "subordinate group status" causes. If you want to talk about more modern causes however, I think you have to consider that currently, subordination is ultimately a choice. When I think of a group as "subordinate" I first think of the work place. Workers are subordinate to their bosses. They choose to accept such terms when they agree to be hired.
Students should be subordinate to teachers (and school administrators), but in our current system, this is one category of subordination that is fought every day, either by apathy, student rebellion, or parents who fight for more student power in a given classroom situation.
I don't honestly see any clear cut situations that lead to subordinate group status in our current society, outside of personal choice to be subordinate, or not.
Historical processes and traditions of oppression lead to subordinate-group status. The trans-Atlantic slave trade lead to the subordinate group status of persons of African descent in the Americas and in Europe. Colonialism often leads to subordinate group status of the natives in colonized lands. Also, the tradition of coverture, in which a man legally owns his wife, lead to the subordinate status of women as a class in parts of Europe during the middle ages and beyond.
A situation where we look at subordinate group status in education is in regards to NCLB data collection. We must report data on how the school as a whole does in regards to meeting standards, but we must also report how specific sub-groups do. We look at sub-groups such as race, special education, low-income, ESL etc. In many schools, it is the achievement status of the sub-groups that determines whether the school as a whole is deemed to have met Annual Yearly Progress or not.
For me, subordinate group status is defined by a group that does not function with the same power as a dominate group (per the dominate groups ideology or societal ideology). Therefore, I would say teacher-student groups, white color/blue collar, and male/female.
One only need look to history for a myriad of examples. When a country has a developed civilization with a well-trained military as did England, for example, it was not difficult for this small country to dominate the uncivilized and undeveloped African continent or the poor and undeveloped country of India during the nineteenth century. Of course, they were a great power with the defeat of the Spanish Armada and dominated the seas for hundreds of years; in the seventeenth century England dominated the subordinate group of the American colonists. Going back to ancient history, those groups that were stronger and better fighters and killers were the dominant groups, were they not. All history is a story of the strong over the weak, whether it is a conquering by physical force or mental acumen.
I don't know that there are exactly three of these. I think that a group might have subordinate status, for example, if it is racially or ethnically different than most of the people in its society. Relatedly, a group might have subordinate status if its national origins are in a country that has been colonized by the country in which the group is living. It may also have subordinate status if it is of a different religion. All of these are situations that lead to subordinate group status.