This is a confusing one, I would agree. Technically, MoiraWendy is correct.
If the subject is plural, then it is 'There seem'.
If you reword the sentence: 'Three principal factors seem to be involved: ...' then it will make sense, because 'There' is an adverb in the original sentence.
However, you will hear: "There seems to be 3 factors involved: ..." all the time. I myself, in speech, will frequently make this 'mistake' as do hordes of other native English speakers!