Powershell for AD account import - need string help

I have a powershell script that I found out on the internet that does a good job of AD account imports but with the amount of accounts I have its current setup will not work for me. I need to include the first three letters of the first name along with the last name for the samaccount to be successful. I have too many duplicate accounts otherwise.  Right now "John Doe" comes out as "DoeJ" but I would like the result to be "DoeJoh"

My current function is this:
Function Set-sAMAccountName {
    Param([Switch]$Csv=$false)
    if(!$Csv)
        {
        $GivenName = $txtFirstName.text
        $SurName = $txtLastName.text
        }
    else{}
    Switch($XML.Options.Settings.sAMAccountName.Style | Where{$_.Enabled -eq $True} | Select -ExpandProperty Format)
        {
        "FirstName.LastName"    {"{0}.{1}" -f $GivenName,$Surname}
        "FirstInitialLastName"  {"{0}{1}" -f ($GivenName)[0],$SurName}
        "LastNameFirstInitial"  {"{0}{1}" -f $SurName,($GivenName)[0]}
        Default                 {"{0}.{1}" -f $GivenName,$Surname}
        }
    }



But if I try to change the below line to include a start and end character the script dies.
  "FirstInitialLastName"  {"{0}{1}" -f ($GivenName)[0,3],$SurName}


Any help would be great!