In Daphne du Maurier's My Cousin Rachel, there's a spurned love interest, the protagonist's childhood friend Louise, who gets fairly little airtime and almost no love from the protagonist, but she keeps cropping up. She provides tension, hope in the reader for a happy outcome, and ultimately is a key part of the surprise ending.
Make a tertiary character meaningful. If you have third-level characters, don't let them lounge around the peripheries like old furniture. Make them figure in the story. Any that don't earn their keep should be reevaluated for either excision or further development.