You could start with travel to Agra. Right from Taj Mahal to Agra fort followed by mehtab bagh, Jama Masjid, Taj museum and Jahangir mahal you could explore the whole day. Night you could shop at Sadaar bazaar and try local delicacies., Next day you could plan travel to Mathura which is 2 hours away from Agra. Starting with Dwarakadish temple, Rangji temple, vishram ghat, museum, bhuteshwar Mahadev temple, Chamundi temple and dauji mandir a day full you could explore. Evening reaching Krushna nagar market you could shop and taste best local delicacies. From Mathura Vrindavan is half an hour away. Starting with Prem mandir, Radha Raman temple, Isckon temple, katyayani Peeth, keshi ghat, Radha Damodar temple, Madan Mohan temple and Shahji temple you could explore a full day. As per your question only 3 days are needed to explore these three spots. If at all you are planning for 5 days, add Jaipur or Alwar or Gwalior which you can cover the rest two days.
1 day is more than enough for Agra....If you are goin for Taj Mahal go early morning since later in the afternoon is alot crowded & a long queue for security & entry
Here is a suggested 5-day itinerary for Mathura-Vrindavan-Agra:
Day 1: Arrive in Mathura and visit the Krishna Janmabhoomi Temple, the ISKCON Temple, and the Dwarkadhish Temple.
Day 2: Take a day trip to Vrindavan and visit the Banke Bihari Temple, Radha Vallabh Temple, and ISKCON Temple.
Day 3: Visit the Agra Fort and the Taj Mahal in Agra.
Also you Can Check- https://www.indianeagle.com/traveldiary/road-trips-from-delhi/
Day 4: Explore the city of Agra, including the Itimad-ud-Daulah Tomb and the Akbar's Tomb.
Day 5: Depart for your next destination.