Whether she was 3 or 300, it is pretty clear that at least some rishonim thought she was 3 and were willing to accept that, and did not challenge it on moral grounds.
I happen not to care how old she was, since it will not change my life, and I will use it neither as a basis to marry a 3 year old, nor as a basis not to.
I can happily accept that it would be a bit odd to marry a 3 year old today, while accepting that yitzchak did it.
This is not the only thing rishonim disagree on, I can live with it.