the list of colleges suddenly got decreases to 3 from 7. what is this step for??

Hii Aditya Gautam..

The management schools which will accept the SNAP scores are considered as the participating institutes. SNAP 2018 was successfully conducted on December 16, 2018 by Symbiosis International (Deemed University). There are 15 participating institutes of SNAP 2018 across India which will accept the scores of SNAP 2018 for the admissions to management programmes. After the declaration of result, all the SNAP participating institutes 2018 will declare their cutoff to shortlist the candidates on the basis of their SNAP scores.

For list check here- https://bschool.careers360.com/articles/snap-participating-institutes

Hope this helps you. All the best.