List.Generate, make behave like Accumulate or XL REDUCE

and try this

let 
f    = {List.Sum,List.Max, List.Average},
head = { "sum", "max", "avg"}, 
val = {2,3,1,5,2,1},
to=
  List.Generate(()=> [ i = 0, j = Record.AddField( [],head {i}, f{i}(val) )  ],
                 (x)=> x[i] <= List.Count( head)-1, 
                 (x)=> [i = x[i] + 1, j = Record.AddField( [],head {i},f{i}(val))
],
                 (x)=>x[j])
in
  Record.Combine( to)