Fetch unique 3 character code

Couple of questions , where you intended to use this and the limit of iteration count ?? This you can write using Code and char function .

Pasting a Sample code for your reference , You need to change the code if the iteration count is more than 25

a!localVariables(
  local!inputcodes: if(
    a!isNullOrEmpty(ri!input),
    null,
    code(ri!input)
  ),
  local!iterate: a!forEach(
    items: enumerate(ri!iterationCount+1),
    expression: a!localVariables(
      local!index:fv!item,
    concat(
      char(local!inputcodes[1]),
      char(local!inputcodes[2]),
      char(local!inputcodes[3] + local!index)
    ))
  ),
  local!iterate
)

Random != Unique.

Very curious as to why you would need to do this but here is some code to generate a string variable length with random characters. Not unique though. You should look into UUIDs if you want to achieve this.

a!localVariables(
  local!character: "A",
  local!numberOfCharactersToAppend: 2,
  local!allCharacters: "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
  local!charactersToAppend: a!forEach(
    items: enumerate(local!numberOfCharactersToAppend),
    expression: a!localVariables(
      local!randomIndex: rounddown((rand() * 26 + 1), 0),
      mid(
        local!allCharacters,
        local!randomIndex,
        1
      )
    )
  ),
  concat(
    local!character,
    joinarray(local!charactersToAppend, "")
  )
)