Hello,
As far as I know the export function in Active Directory Users and Computers is limited however in PowerShell you can do it quickly via this cmdlet from a computer or server with the Active Directory PowerShell Module
Get-ADUser -Filter * | Select-Object Name,UserPrincipalName,SamAccountName
To export it simply add the Export-csv at the end like this
Get-ADUser -Filter * | Select-Object Name,UserPrincipalName,SamAccountName | Export-Csv -Path export.csv -Delimiter ";" -NoTypeInformation -Encoding UTF8
Regards,