Find 3 character smtp prefixes

I'm trying to list all the users in my domain whose primary SMTP Prefix is 3 characters.  I thought I could just do:

Get-QADUser -sizelimit 999999 | where {$_.PrimarySMTPPrefix -like "???"} | Select-Object name, primarysmtpaddress

However that doesn't seem to work.  I've also tried:

Get-QADUser -sizelimit 999999 | Select-Object primarysmtpaddressprefix | Measure-Object -property length

But that doesn't seem to output anything.  Any help would be appreciated.

Thanks,

Isaac