Not tested fully but this should get you what you want.
#!/bin/sh
computerName=$(hostname)
echo "$computerlName"
beginHostname=${computerName:0:3}
echo "$beginHostname"
if [ "$beginHostname" = "MBP" ];
then
echo "Computer Name is already set to: $computerName"
exit 0
else
echo "Compuer Name is not set. Prompting to enter Asset Tag"
assetTag=$(/usr/bin/osascript <<EOT
tell application "System Events"
activate
set assetTag to text returned of (display dialog "Please type in Asset Tag if known. Otherwise click cancel" default answer "" with icon 2)
end tell
EOT
)
fi
if [ "$assetTag" == "" ]; then
echo "Asset Tag unknown. Setting Computer name to Serial number"
name=$(system_profiler SPHardwareDataType | awk '/Serial/ {print $4}')
else
echo "Setting computer name to Asset Tag"
name="$assetTag"
fi
echo $name
networksetup -setcomputername $name
scutil --set LocalHostName $name
scutil --set HostName $name
sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.smb.server NetBIOSName $name