Why are zofran, ativan and Benadryl (diphenhydramine) the main three drugs used for nausea and vomiting on pediatric oncology?

They work well... Every patient is different with respect to the best combination of anti-nausea medicines that works for them. We tailor an anti-nausea regimen based on the patient and what chemotherapy they are receiving. There are other medications that can be added to the ones you mentioned, as well as acupressure and acupuncture (applied safely) that can help patients.
MANY OPTIONS. First, smart oncologists are using these and others transdermally--through the skin. Zofran (ondansetron) is one of the oldest serotonin nausea center blockers. Ativan calms and relaxes muscles--either/both can decrease stomach issues. Benedryl has anti-histamine properties that at times lowers inflammation and nausea. Many other options exist and a parent's goal should be to be full comfort.