As its standalone game, it don't have to be short, and possibility to play more than 3 rounds could provide more tactics etc. Of course in that case you would start with more cards.
Did you guys have tested something like this?
Well the shorter the games the better it is suited for mobile. And gwent would be great for mobile!
So in that way having max 3 rounds is preferable.
Also if you need to win 3 times rather then 2 it would need up to 5 rounds.
For that either the number of cards have to be increased, or you just play 0-1 cards in the rounds you want to lose, which is kinda boring.
When you need more hand cards, the total number of cards also needs to be increased, and the possibility of different decks will decrease.
So all in all I do not realy see why 5 rounds should be better. With 3 rounds you can allready quite well try to deceive your opponent (you can lose 1 round) more is not that much needed, and you will not be able to trick the enemy 2 times into playing more cards then needed. (Especially as you have a limited number of spies and the swap cards).