This time, it's a large number of specific applications of a single engine family.
Last time, let's see:
LH family (so named, but the 3.3 came before the LH) -- 3.3, 3.5, 3.8, 4.0
"AMC" truck engine -- 3.7
"6/8 of a 360 V8" engine -- 3.9
MMC 3.0 and 2.5
Why?
Well... first came the Mitsu 3.0. Chrysler was going to buy all of Mitsubishi, so that made sense -- their future division made an engine that they needed, so they bought it for the few vehicles that really needed them. As time went on, it must have dawned on people that they could build something better suited to their needs, cheaper, and that fuel would not be getting scarce again any time soon, so V6 demand would stay.
So then the 3.3 was born. It met their power needs, sitting above the 3.0, and complementing it. They still bought the 3.0, partly to avoid investments in extra production capacity, I suspect. Eventually the 3.0 was replaced by the Mitsu 2.5 V6 which was mainly used in cars made by Mitsubishi. (I think the early cloud cars used it, too, probably because of its small size -- it fit.)
The 3.8 came when they needed more power than the 3.3 could muster, and wanted a premium engine ... so a long-stroke. Then the 3.5 was developed fresh, but with the same basics, as a very modern, powerful engine for the LH to show the new era. Eventually we got the revised 3.8 and the new 4.0 based on that family.
The 3.9 came, I think, before the 3.3, because they needed something with more power for the Dakota. It was a quickie, so to speak, there wasn't money for a new engine, so they cut two cylinders off the 360 (they used 'em later to make truck V10s.)
Then the 3.7 was designed as a truck engine, off the 4.7 model, to replace the 3.9.
Now, the 3.6 came in for SUVs and minivans and was used in cars as well; it's a good general purpose engine, it seems, with different tuning and some internal parts changes for various adaptations, e.g. Wrangler and Ram. But they need something smaller and lighter and more fuel efficient for Cherokee, and the 3.2 was born; it is also good for the cars and will probably replace the 3.6 in some applications, because it provides most of the power with better mileage.
Then, again, the 3.0 is designed for Europe.
That isii the high level overview of my understanding of the situation. If you want a lot more detail, get the Weertman book.
http://www.allpar.com/reviews/other/engines.html