With an SC rank of 594 in IIST and a current waiting list position of 438 for M.Tech after the third round, your chances of getting a seat are quite low, but not entirely zero.

IIST has a limited number of M.Tech seats, and the number of candidates who move up the waiting list depends on how many people above you withdraw or don’t accept their offers. In most cases, waiting list movement beyond 100–150 ranks is rare, especially in specialized institutes like IIST where the intake is small and candidates are highly interested in keeping their seats.

Since your waiting list number is 438, and you're in the SC category, your chance would mostly depend on massive withdrawals or unfilled seats, which is unlikely this far into the counselling rounds unless there are further special rounds or spot admissions announced.

It’s always good to stay in the process till the end, just in case more rounds or spot rounds are held. Also, keep exploring backup options in other institutes or through CCMT if you haven’t already.