if文のサイト等で中身を見ましたが分かりませんでした。

どこまでわかって何がわからなかったのでしょうか？

普通にif文かけますよ。

 "NULL,hoge,2"⇒"　,hoge,2"

Get-Content "[ファイルパス＆ファイル名.csv]" | % { 
    if (($_.Split(",")[0] -eq 'NULL') -and ($_.Split(",")[2] -eq '2')){
        "　" + "," + 
        $_.Split(",")[1]  + "," + 
        $_.Split(",")[2]  + "," 
    } else {
        $_.Split(",")[0]  + "," + 
        $_.Split(",")[1]  + "," + 
        $_.Split(",")[2]  + "," 
    }
} | Out-File -Encoding default [ファイルパス＆ファイル名.csv]

 ",hoge,2"⇒"　,hoge,2"

Get-Content "[ファイルパス＆ファイル名.csv]" | % { 
    if (($_.Split(",")[0] -eq '') -and ($_.Split(",")[2] -eq '2')){
        "　" + "," + 
        $_.Split(",")[1]  + "," + 
        $_.Split(",")[2]  + "," 
    } else {
        $_.Split(",")[0]  + "," + 
        $_.Split(",")[1]  + "," + 
        $_.Split(",")[2]  + "," 
    }
} | Out-File -Encoding default [ファイルパス＆ファイル名.csv]

 テスト結果

PS C:\tmep> (',hoge,2', 'NULL,hoge,2', 'fuga,hoge,2')|%{
>>     if (($_.Split(",")[0] -eq 'NULL') -and ($_.Split(",")[2] -eq '2')){
>>         "　" + "," +
>>         $_.Split(",")[1]  + "," +
>>         $_.Split(",")[2]  + ","
>>     } elseif (($_.Split(",")[0] -eq '') -and ($_.Split(",")[2] -eq '2')){
>>         "　" + "," +
>>         $_.Split(",")[1]  + "," +
>>         $_.Split(",")[2]  + ","
>>     } else {
>>         $_.Split(",")[0]  + "," +
>>         $_.Split(",")[1]  + "," +
>>         $_.Split(",")[2]  + ","
>>     }
>> }
>>

　,hoge,2,
　,hoge,2,
fuga,hoge,2,

Import-Csv, Export-Csv という便利なコマンドレットがありますよ。
仮に CSV が abc.csv という名前で、次のような内容だったとします。

abc,def,ghi
0,1,2
3,4,5

この時、このように読み込みます。
「PowerShell>」はプロンプトです。

PowerShell> $a = Import-Csv -LiteralPath abc.csv -Encoding UTF8
PowerShell> $a
abc def ghi
--- --- ---
0   1   2
3   4   5

加工します。

PowerShell> $a |% { if ($_.abc -eq 0) { $_.abc = '99' }; $_ }
abc def ghi
--- --- ---
99  1   2
3   4   5

読み込み、加工し、保存します。

PowerShell> Import-Csv -LiteralPath abc.csv -Encoding UTF8 |
>> % { if ($_.abc -eq 0) { $_.abc = '99' }; $_ } |
>> Export-Csv -LiteralPath dst.csv -Encoding UTF8 -NoTypeInformation

追記
PowerShell の条件式

例: 読み込んだレコードのフィールド abc が空文字で def が 2 かどうかを判定し、真なら abc を全角空白に差し替える

if ($_.abc -eq "" -and $_.def -eq "2")
{
    $_.abc = "<全角空白>"
}