wht i have read and understand is tht all 3 gods wasnt able to kill madhu and kaitabh...they were unbeatable...the rakshas...and durga ji kee artee it self says who kiled madhu and kaithabh
madhu kaitabh dou maray
sur bhay heen karay
jai ambay gouree
You have knowledge of Ramayan, but I know DSS, DeviPuran, Devi Bhagvat, Shiv Puran, Mahavidyas & more too.
Durga-Saptashati 1st Charitra-Mahakaali- Brhmaji prays to Mahakaali, that prayer is also called the Ratri Sukta. Vishnuji awakens, he fights with Madhu, Kaitabh for 5000 years finally kills them. 2nd Charitra-MahaLakshmi- All Gods collective energy creats Ma Durga, who kills Mahishasur.
3rd Charitra- MahaSaraswati- Chand, Mund, Raktabeeja, Shumbh, Nishumbh are killed.
What you have mentioned is just a part of hindi translation. Read the Sanskrit version to know the real story.