Fallacy of a single cause

My question is,  is the fallacy of a single cause, the same as reductionism, or reductivism?

The fallacy you name - that of the single cause - is a form of reductionism called causal reductionism, a fallacious argument within a debate regarding causation of a given factor.

'Reductionism' (I'm not familiar with the word 'reductivism'!) by itself implies a broader concept, the applications of which may or may not be fallacious. It is probably better described in terms of something else, like a person's mindset (e.g. "he's a pretty reductionist thinker"). To put it simply, a reductionist sees "the whole as the sum of its parts".

There is the fallacy causal reductionism (also known as fallacy of single cause) as well as the oversimplified cause fallacy. Very often, words used in fallacy names also have legitimate uses. For example, the concept of the stereotype is legitimate heuristic but can also be fallacious, as in stereotyping (the fallacy) . Likewise, there are legitimate slippery slopes and the slippery slope fallacy. It is perfectly fine to break down a complex system for understanding, but the fallacy would occur in an argument context when oversimplification is involved.

As for your specific question about naming, I wasn't familiar with the term "reductivism" but it seems it is used to mean an oversimplification, although not an "official" word (if that matters).