Below Powershell 7.2 answer should work. Returns the correct answer locally, but returns 0 on here.

function maxTriSum ([int[]]$numbers)
{
  [int[]]$intArray = @()
  
  $numbers = $numbers | sort -descending
  
  for($i=0;$i -lt $numbers.length -and $intArray.length -lt 3;$i++)
  {
    if($numbers[$i] -ne $numbers[$i -1])
    {
      $intArray += $numbers[$i]
    }
  }
  
  return ($intArray | Measure-Object -sum).sum
}