Ummm . . . every 3e game I ever played in featured combat against multiple enemies. Guess we were doing it wrong.
There are a lot of factors involved in what numbers of enemies tend to work.
You might ask yourself this:
- How many opponents where there usually? 1-4, or 3-12?
- What CR ranges where involved?
- How difficult did this encounters feel?
- How tricked out where your characters?
- How many of them did you have per day?
- How much time did you spend on these combats?
And then compare them to the answers for 4E. I would say the overall tendency would go towards "more monsters" "either higher level then it used to be, or more per day", "shorter durations", "challenges keep constantly (high) within a wider range of encounters per day / levels)
While the 3E encounter "calculation" system was based around one monster vs. one party of equal level, this didn't mean that was the standard encounter, too. Of course, most of the mechanics and CR rates where build around these guidelines, which means diverging from the "mathematical standard model" of encounters lead to certain side effects that were often not seen positive. 15 minute adventuring day or many "too easy" encounters. (But since the latter was usually perceived as a boring experience, I think the short adventure day become more common. Even if "unrealistic", it still meant a higher degree challenge and entertainment.)