Insert different table row or list range at each positon

List.Generate versions if interested.

This iterates the inserts and then grabs last one as output.

let
    alist =  {"A".."Z"},
    addPosition = {3,7,12}, // buffer if query output
    addVals = {1,2,3}, // buffer if query output
    genAdds = List.Generate(
        ()=>[ i = 0, l = alist ],
        each [i] <= List.Count(addPosition),
        each let curIndex = [i] in [
            i = curIndex+1,
            l = List.InsertRange( [l], addPosition{curIndex}, {addVals{curIndex}} )
        ],
        each [l]
    ),
    output = List.Last( genAdds )
in
    output

And here is a variation where inserts retain original positioning.

let
    alist =  {"A".."Z"},
    addPosition = {3,7,12}, // buffer if query output
    addVals = {1,2,3}, // buffer if query output
    addCount = List.Count(addPosition),
    genAdds = List.Generate(
        ()=>-1,
        each _ < addCount,
        each _ + 1,
        each [
            isLast = _+1 = addCount,
            curPos=if _ = -1 then 0 else addPosition{_}, 
            nextPos=if isLast then List.Count(alist) else addPosition{_+1}, 
            count = nextPos-curPos, 
            output = List.Range( alist, curPos, count ) & ( if isLast then {} else {addVals{_+1}} ) 
        ][output]
    ),
    combineParts = List.Combine( genAdds )
in
    combineParts