Are there any good editions of texts available online or offline that provide a side by side, comparative study of the commentaries on the Prasthana trayi (Upanishads, Brahma Sutras and Bhagavad Gita)? I am of course looking for 3 different texts, separately for each text. Links to book titles or actual file would be very much appreciated.
Following are some books on Brahma Sutra comprising analysis of principal commentaries of Adi Shankaracharya (Advaita), Sri Ramanujacharya (Vishishtadvaita and Madhvacharya (Dvaita) available on Exotic India:
A Glossary of Technical Terms in The Commentaries of Sankara, Ramanuja and Madhva on The Brahma - Sutras (Set of 2 Volumes)
The Vedanta (A Study of The Brahma – Sutras with The Bhasyas of Samkara, Ramanuja, Nimbarka, Madhva and Vallabha)
The Brahmasutras and Their Principal Commentaries A Critical Exposition (In Three Volumes)
For Srimad Bhagavad Gita, you can refer bhagavad-gita.us where commentaries of Acharyas of various philosophies or Sampradaya are provided simultaneously verse-by-verse. Also take a look at Gitasupersite where you can find various English translation and commentaries verse-by-verse.
For Upanishads, I've not found any source where all commentaries are comparatively provided. You can easily find Adi Shankaracharya's Bhashya on principle Upanishads on Internet Archive, Exotic India or from Wisdom Library for Advaita and Rangaramanujacharya's Bhashya for Vishishtadvaita, see vol 1, vol 2 and vol 3.
For Bhagavad Gita, you may want to check out the work of T.G. Mainkar
The book is
A comparative study of the commentaries on the Bhagavad Gita by T.G. Mainkar
I believe it was published by Motilal Banarsidas, though I don't know if it still remains in print. http://www.mlbd.in