I do agree that destruction causrd by three 200KT nuke will be roughly equal or just above 1 MT device. But problem with india isBefore this is discussed lets first talk few basic points. I guess you might already know this but lets talk about it anyways.
1. Damage drops inverse inverse cubic relation of distance from the ground zero. Meaning, from a place that is 10 KM away from ground zero will have one thousandth of damage than a place which is one kilometer away from ground zero.
2. So, it is better to have multiple smaller bombs than one single megaton bomb dropped on a city for instance.
Based on this, no, India will not even attempt to go for a megatonne yield bomb. Its not needed. India will like to have 100-250 KT yield bomb. Now is this hydrogen bomb? Well... no one knows. There are doubts about our design of hydrogen bomb. But we may just be able to deliver weapons without even having a hydrogen bomb design working.
India indeed tested a plutonium based boosted fission bomb. This can scale easily to 100 Kilotonne or may be 200 kilotonne. At 100-120 Kilotonne, it can weight (in best cases) upto 500-700 KG. A comparable design will be french MR-41 warhead from 60s-80s. It had a yield of 500KT and weigh 700 KG. I am sure, Indian scientists should have figured how to put together a 100-120 KT yield warhead with a weight of 500-700 KG.
So you have a 600 KG (mid-figure) warhead with yield of 100 KT. This if MIRV'ed can fit 2-3 warhead in missile nose Agni-3 modified to have a MIRV design. So with 2-4 missile per city, you can blow up entire cities like Shanzen or Lahore. With 10-15 missiles you can blow up 4-6 cities of China or Pakistan.
The cost is launching 2-4 missiles where one missile would have been enough and using more nuclear fuel.
1) we yet to master MIRV technology, and it is minimum 5-10 years away fro a functional MIRV ( based on our previous record of roughly 5 year development cycle for a new ballistic missile ). So we need to use precious A5 for single weapin delivery in numbers to match chinese megaton H bomb.
2) We don't have a long range bomber to carry few nuke devices over china.
3) The biggest concern is, we dont have enough inventory of nuke to wipe out china, reason is we need to take care pakistani too.
So i feel we should start building megaton H bomb, it may not be an efficient killing machine but surely we can save few ballistic missile.