ಮೈಸೂರು - ವಾರಣಾಸಿ ಎಕ್ಸ್‌ಪ್ರೆಸ್: ಕಾವೇರಿ ಮತ್ತು ಗಂಗಾ ನದಿಗಳ ಸಂಪರ್ಕ ಸೇತುವೆ - Vishwanath Joshi

You may be right,but 3rd platform was used for mainline trains as well,
I think 3rd platform may had metre gauge track for bvrt line as it was a meter gauge before 1960s so they made it dedicated for bvrt trains,but years back bvrt side trains are always arriving on pf 1 and 2 only in recent times due increased traffic they started using 3rd pf for bvrt trains 1,2 platforms for bvrt trains are only used when DEMUs occupied 3rd Platform and only when mainline is free,if 3rd platform is directly connected to bvrt line why don't they used it in first place,
If...
more...

Platform 3 is occupied by DEMUs bvrt side are held at outer to make way for tdd side trains because bvrt side trains are mostly normal exp trains with low prioritiy
NDD needs 2 new platform otherwise the situation will be similar to Gudur Jn,only after new pfs Gudur became a better station and trains entering pfs rather than waiting at outer for 10,15 minutes they can use 4,5 pf for bvrt trains so that they can wait at station itself