Regarding DN/DA (長阿含), Ven. Yin Shun states it was developed and expanded from the Geya (祇夜) anga portion of SA/SN:
The original SA/SN sutras were called the ‘Connected Discourses’ (相應教, saṃyukta-kathā, the synthesis of the three angas).
See p. 899, note 21, in Choong Mun-keat, “Ācāriya Buddhaghosa and Master Yinshun 印順 on the Three-aṅga Structure of Early Buddhist Texts ”, Research on the Saṃyukta-āgama (Dharma Drum Institute of Liberal Arts, Research Series 8; edited by Dhammadinnā), Taiwan: Dharma Drum Corporation, August 2020, pp. 883-932.
https://www.academia.edu/44055729/%C4%8 … hist_Texts
It is likely that the SA/SN sutras were called as a Agama or Nikaya “collection” until the other three Nikayas/Agamas (MN/MA, DN/DA, AN/EA) were gradually developed and expanded from the ‘Connected Discourses’ (相應教 , saṃyukta-kathā ) in Early Buddhism.
This information is also indicated in the above-mentioned article note 21 from both the Chinese and Tibetan texts. The Chinese text reads:
“即彼一切事相應教 間廁鳩集。是故說名雜阿笈摩
即彼相應教 。復以餘相處中而說。是故說名中阿笈摩
即彼相應教 。更以餘相廣長而說。是故說名長阿笈摩
即彼相應教 。更以一二三等漸增分數道理而說。是故說名增一阿笈摩”
相應教 (saṃyukta-kathā) came first and then 雜阿笈摩, … 長阿笈摩, 增一阿笈摩 in the formation of EBTs in Early Buddhism.
Cf. also: pp. 9-11 in Choong Mun-keat:
The Fundamental Teachings of Early Buddhism: A Comparative Study Based on the Sūtrāṅga portion of the Pāli Saṃyutta-Nikāya and the Chinese Saṃyuktāgama (Series: Beitrage zur Indologie Band 32; Harrassowitz Verlag, Wiesbaden, 2000).
https://www.academia.edu/12359515/The_F … ukta_agama