Learn English – Using “because”, “as”, or “since” when explaining the reason or relevance of something in an adverbial clause

Here's my opinion of your alternatives.

1. "because" is your best choice for clear, correct, unambiguous communication, especially if you want to be understood easily by other non-native speakers.

2. "since" is acceptable, although it makes your statement ambiguous; the second meaning would be that statement A became true at the time that statement B became true. Example: "I study more often since I enrolled in class" conveys that when you enrolled in class, then you began studying more often. Rule of thumb: prefer "since" when your intent is to convey "from the time that".

3. "as" is somewhat acceptable, but in your particular statement structure "as" has more useful meanings of "abstractly analogous" or "synchronously". Example: "I like apples as you like oranges" conveys that my liking is similar to your liking, i.e. equivalently strong/weak/notable/etc. Example: "I make dinner as you set the table" conveys that we do these tasks at the same time in the same place. Rule of thumb: prefer "as" when your intent is to convey "sameness".

4. "for" can be acceptable, although I would never expect to hear it in normal conversation. It would come across as overly-academic, or possibly epic or religious. Example: "We eat well tonight, for tomorrow we go to war." Rule of thumb: prefer "for" when you want to be poetic and inspirational.

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.