Is it permissible to abandon a matter that Allah has made easy after performing the Istikhara prayer, for a third matter that was not a subject of hesitation?

Istikhara (seeking guidance from Allah) is recommended for a Muslim if they are hesitant about a permissible matter. It is not performed for obligatory or recommended acts (because one is commanded to do them), nor for forbidden or disliked acts (because one is commanded to abandon them). If a person performs Istikhara between two matters and then does a third, there is no harm in that, for that might be what is best for them.