The Ottoman Empire has largely replaced the Byzantine hegemony in the Balkans and Near East by the 1399 start date, and looks poised to capture the last Greek holdings, namely Constantinople (later Istanbul).
While the Ottomans are riding on the momentum of many successful campaigns against the Byzantines, historically they are actually on the eve of Timurid invasion in 1402 which would lead to the capture of the Sultan and a decade long civil war.
Very rarely does Europa Universalis take that course of action however. In the first years of the game, the Ottomans normally consolidate their position in Asia Minor by incorporating the Turkish minors into their realm.
By the late 1400's, the Ottomans begin expanding northeast in most games, into the Caucasus and around the Black Sea, around the same time as the historical Ottoman Empire was preparing to go to war with Persia. Constantinople is rarely conquered by 1453, but rather closer to the turn of the century, if conquered at all. In fact, close to 50% of the time, another Turkish minor or Austria strikes first.
By 1600, the Ottomans have typically reached their peak, looking a little ridiculous, resembling the western half of the Mongol Empire. This I think highlights a flaw in the Europa Universalis AI; typically they conquer what is easiest, not what is most beneficial to them.
From 1600 onwards the Ottomans normally begin to collapse into themselves, having greatly overextended themselves. This collapse is most often precipitated by Austria, who themselves often collapse in a similar manner. Still, in the minority of games, the Ottomans manage to fulfill their weird dream of setting foot in the Pacific Ocean. Maybe they envisioned a Turkish Alaska?
The famous House of Habsburg takes the helm of Austria in 1399, which historically was a house divided at the time, with much bickering and dispute between family members. As you'll find out, this isn't the case in Europa Universalis III; Austria quickly becomes a great power, if not a superpower.
Most games begin with the absorption of Hungary, which historically doesn't happen until after the Battle of Mohács in 1526, when it was inherited along with Bohemia by Ferdinand I.
You will find that the Europa Universalis Habsburgs are much more bloodthirsty and impatient than the historical Habsburgs, as by the late 1400's they often begin to push into the Ottoman Empire. The is completely reversed from history, with the Ottomans very nearly conquering Austria during the Siege of Vienna in 1529.
By the mid-1500's, one can forget entirely about history as the Austrians begin to form a nice blob in the Balkans. Their holdings, depending on the game, will stretch anywhere from France, to Persia, to Siberia. It is not uncommon for the outer reaches of the Austrian Empire to fall into rebellion, then be reconquered, and cycle through these two states many times during the game.
Significant colonisation in most games begins by the mid-1600's along the Brazilian shore. By the early 1700's the Austrians begin moving into North America, normally focusing on New England and later the American South and Mexico. It is important to to note that all historical Austrian colonial efforts were insignificant and limited to just a few islands, all of which eventually failed.
By the game's end date, 1820, much of the globe is apparently speaking German. I would argue that, in Europa Universalis III, Austria most often becomes the world's strongest nation. That being said, they are typically also the most unwieldy and unpredictable nation, making for many interesting games.
The King of Bohemia, Václav "the Idle" (who is historically deposed in 1400), begins the game as the Holy Roman Emperor. Things are looking up for Bohemia, a land of Czechs and Germans.
Bohemia's first moves are typically expansion into Poland, after which they continue slowly making their way east. Bohemia, compared to their eastern neighbours, is much more technologically advanced in 1399, making expansion an easy prospect.
Historically, Bohemia would become embroiled in many regional conflicts due to their position in the Holy Roman Empire, but their borders were relatively stagnant. Despite this, in many games, Bohemia continues their expansion into northern Hungary in 1500.
Historically, as mentioned previously, Ferdinand I of Austria would inherit Bohemia in 1526 after the Battle of Mohács, and Bohemia would remain Habsburg up until the conclusion of the First World War. In by far the majority of games Bohemia will defy this course of history, and would expand even further, often reaching Lithuania and the Golden Horde by the year 1550.
From the 17th century onwards, Bohemia's borders would remain relatively unchanged. They appear to make frequent eastern excursions, most of which eventually fail. Bohemia very rarely undertakes any colonial endeavours, and effectively stays put until the game's end, content with their little empire spanning the plains of Eastern Europe.
With many of the major European powers already covered, I think I will focus on a particular region, or perhaps the East Asian or Indian nations next week. Please leave a comment and let me know what you would like to see!