CC3 characters were indeed released when CC3 first came out, but it lacked certain features.
CC3+ was a more complete version of the character with many advances, including SkinGen and better topology and facial features. There is also support for AccuLips.
Rather then trying to list that all here, please refer to this thread:
https://forum.reallusion.com/450126/02-New-Character-Base-CC3-Base-.
CC4 brought a further improvement of facial features of the character, but is still called a CC3+ character. It will support a wrinkle system in the near future.
So as a developer you would best concentrate on CC3+ characters. However, for compatibility with iClone 7, you would need to develop them with CC3, not CC4.