Hello,
Currently an extension button can be defined with one external phone number which in turn is associated with mobile handset icon.
Please, consider, adding one or two more buttons, each matching with external2 or external3 supplementary settings.
Regards
You can use the phonebook for that, cluttering action buttons is not an option.
Add phonebook entries for them, then, when on a call, type the person name on the dial box and choose the number you want to transfer from the select box and hit enter.
Best regards,