А fellow wants to spend some time at the beneficiary of a Global Fellowship (the European return host). How many months are left for the outgoing phase if they choose to stay 3 months at the beginning of a fellowship?

The return phase always lasts 12 months, there are no exceptions. Starting at the European institution counts as outgoing. This means that if the outgoing phase is 24 months, only 21 months will be left after the secondment at the European return host.