Moved the modulus check into the scriptblock

Leaner.ps1

[int]$FizzRunningTotal = 0
[int]$FizzBuzzRunningTotal = 0
[int]$BuzzRunningTotal = 0
$Mod15 = [Scriptblock]::Create('param([int]$Number) if ($Number % 15 -eq 0) { $FizzBuzzRunningTotal + $_ }')
$Mod5 = [Scriptblock]::Create('param([int]$Number) if ($Number % 5 -eq 0) { $BuzzRunningTotal + $_ }')
$Mod3 = [Scriptblock]::Create('param([int]$Number) if ($Number % 3 -eq 0) { $FizzRunningTotal + $_ }')
1..15 | ForEach-Object {

    $FizzRunningTotal = $Mod3.InvokeReturnAsIs($_)
    $BuzzRunningTotal = $Mod5.InvokeReturnAsIs($_)
    $FizzBuzzRunningTotal = $Mod15.InvokeReturnAsIs($_)

    [PSCustomObject]@{
        Number     = $_
        FizzRunningTotal = $FizzRunningTotal
        BuzzRunningTotal = $BuzzRunningTotal
        FizzBuzzRunningTotal = $FizzBuzzRunningTotal
    }   
} | Format-Table -AutoSize