I would imagine this is just because there has been no pre-season. Hardly a massive issue to allow fresh legs, and the rules have already changed because the season was suspended and is now re-starting.
For me, it is down to the players now, not any moaning about how unfair all this is, life is unfair, the virus has been unfair. The imbalance is there, whether 3 subs or 5. I wouldn't have made the change personally but I assume they all agreed to this and as said, just for match fitness.
Do we think we would stand that much more chance if we only used 3 subs? I don't. I get what you are saying, but it looks a meh change to me.