untypical, atypical, nontypical

Atypical is by far the most common of the three, as confirmed in a Google Ngram search, so that would be my suggestion.

Untypical is apparently most often used in the phrase "not untypical". Another Google Ngram search supports this.

Nontypical, and with a hyphen non-typical, are hardly used at all. I cannot find an entry for either in any popular online dictionary.

This Ask the Editor response has more details, which basically echo what I said. I'll quote it below:

Because the prefixes a- and un- can both mean not, atypical and untypical have the same meaning: not typical, not usual or [not] normal. However, the contexts in which these two words are used are different. There is also a 3rd choice with the same meaning, which will be discussed below.

1. atypical Atypical is the most common of these choices. However, it is used mostly in formal language about medical topics, as in these examples:

• These cases were atypical because the patients were diabetic.

• The biopsy showed a few atypical cells.

2. untypical Untypical is used much less often than atypical, and it is becoming rarer. When untypical is used, it is most often after the word not, as in this example:

• The summer weather arrived suddenly, which is not untypical for New Orleans. (not + untypical = typical)

3. not typical The phrase not typical is used more than untypical. Not typical can be used in a variety of contexts to talk about all kinds of topics, as shown in these examples:

• This was not typical behavior for a 10-year-old.

• It was not a typical business meeting.

• This is not typical weather for Miami.

In US, for sure we have all terms ; even more we need or know... In English professional litterature (sciences ; law) we use these terms.

Atypical : is for no possible relevant type ; it's the statistical accident among all possibilities. It's a final & closed evaluation.

Untypical : would mean we find it but not in enough cases which would yet deserve a particular type (but it could...). This explains in your question "temporarily untypical"

Nontypical : means you're quite sure of your previous identified types & won't add nor need for your work another type/class to let your observation outside of the previous created categories, which are maybe not perfect but appropriate for what you're doing. It's not so peremptory like what you must have in statistics ; it's what you have in probabilities & sufficient for such last purpose.

But every day we don't make these distinctions...