Hello.
How many 3 letter combinations would there be in the alphabet? They’re can’t be repeating combinations (ADG, GDA, DAG) it doesn’t matter what order they are in, there just can’t be the same 3 letters in any orientation in a combination of 3 letters.
Hope this makes sense, thank you!
Your wording is awkward, so let's clarify it.
When you say, "there can’t be repeating combinations (ADG, GDA, DAG) it doesn’t matter what order they are in", you mean that you want to count
distinct combinations, ignoring order. "Repeating" sounds as if you meant that no letter within a combination can repeat, but that appears to be covered by your next clause instead, "there just can’t be the same 3 letters in any orientation in a combination of 3 letters". But this, taken literally, would only exclude, say, AAA, in which all letters are the same, and not cases like AAB where only two letters are the same.
But it does appear that pka is answering your intended question; "combinations", as a technical term, means distinct subsets, that is, sets of (3) letters with no repetitions, ignoring order.
For information about the concept and how it is calculated (and, more important, why), see
https://www.mathsisfun.com/combinatorics/combinations-permutations.html; for a more formal explanation, see
https://en.wikipedia.org/wiki/Combination.
