Russia most likely. India did it during Cold War and will do it again. US knows this.
India during Cold War =/= India now. India during the '60s-'80s era was a closed and a heavily controlled economy that was heavily reliant on Soviet aid. We used them as a crutch so that the government could continue being the sole source of provisioning barring about 3-4 sectors. You could, therefore, say that the cooperation between India and the Soviets was the result of economics. The minute the USSR showed signs of imminent collapse in the beginning to middle of the year 1991, the Indian economy fell into hot water due to the balance of payments crisis, something which we were only able to crawl out of thanks to the Indian government accepting Washington Consensus.
From that point on up to now, India has been reliant on the Western markets and especially the United States. We have literally 0 reasons to support Russia other than the fact that a good chunk of our military hardware comes from there (something we've been actively working to minimise) as well as the fact that we are currently making bank by selling refined Russian oil. Economically speaking, our trade with the US is much more profitable because we hold our largest trade surplus with them and it has the potential to only grow more than 10 times. There is also the fact that the Indian state receives about a quarter of it's remittances from the US. We have no reason to extend open support to the Russians over the Americans at this point in time.
Besides, you will still have to consider that siding with Russia would in turn mean siding with China and given the difference in sizes of Indian and Chinese economies, it would effectively mean that we would accept Chinese hegemony in Asia and that is absolutely unacceptable to the Indian government due to the fact that the Chinese claim Ladakh and Arunachal Pradesh and there has been a history of China using it's military to intimidate India into giving up territory. There is also the fact that the Chinese have the practice of dumping their products into India which has time and again shown adverse effects to local Indian manufacturers. Until and unless these issues are resolved (which it absolutely won't in at least the foreseeable future), I just don't see India siding with the Russia-China-Iran axis.
With that said, I doubt if even India would enter the conflict openly at all unless provoked at first by an aggressor. The best course of action would be to stay neutral but apply pressure on the Chinese during a hypothetical invasion of Taiwan to resolve the border issue once and for all. After dying for someone else's interests for the last 2 world wars, Indians should, for once, look after their own.