Hello Kiritvankadi

All 3 of them are excellent colleges with very good ratings and infrastructure and faclilities but the difference in fees is  very great for IIT Ropar the fees is Rs 8.60 lakh for 4 years,for IIST the fees is Rs 1.8 lakh for 4 years but if you keep scoring above 8 SGPA  you don't have to pay any academic fees ,for BITS Pilani the fees is Rs 16.5 lakh for 4 years and in terms of placement the median salary for IIT Ropar is Rs 11.20 lakh per annum, for IIST it is 9.15 lakh per annum and for BITS it is  Rs 13 lakh per annum.So you choose a college according to the stream you will like to do I will also advise you to compare the colleges using the college compare and check out by yourself i will provide the link for that i will even provide the link for all 3 colleges so you can check them out in details which will help you even more

https://www.careers360.com/compare-colleges

https://www.careers360.com/university/indian-institute-of-technology-ropar

https://www.careers360.com/university/indian-institute-of-space-science-and-technology-thiruvananthapuram

https://www.careers360.com/colleges/bits-pilani-hyderabad-campus-hyderabad

hope this helps you