With White, Ivanchuk chose Fischer's 6.Bc4 against Nepomniachtchi's Najdorf, but got nothing. If anything, it was his opponent who had some chances, but apparently neither player felt very comfortable and a draw was agreed on move 19.
Mamedyarov-Gustafsson didn't last much longer, and like the Ivanchuk-Nepomniachtchi game, it ended too soon. With 26.Be2 (instead of repeating with 26.Qe2) Mamedyarov seemed to have a promising position, but he disagreed and called it a day.
The third game to finish was in a sense the first one to end. Naiditsch produced a very clever novelty against Kramnik's Petroff, 19.Qd2. The move offers a full rook, but its real value was psychological. It's often possible to decline a sacrifice and achieve reasonable play, but this was not one of those occasions. After 19...Ng6? 20.Ree1 f6 21.Rad1 Kf7 22.Qe3, White was clearly winning. Kramnik gave up his queen for a rook and knight, hoping to achieve a fortress. It seemed unlikely to succeed, but with a blunder on move 41 he didn't get the chance to find out.
Last but least, the supremely dull van Wely-Leko game yawned to a finish.
Games here, with my comments.
Standings after Round 3:
1-2. Gustafsson, Leko 2
3-6. Kramnik, Naiditsch, Nepomniachtchi, Mamedyarov 1.5
7-8. Ivanchuk, van Wely 1
Pairings for Round 4:
Kramnik - Leko
Nepomniachtchi - van Wely
Gustafsson - Ivanchuk
Naiditsch - Mamedyarov
At least, this is my assessment, based on the assumption that Naiditsch's novelty (19.Qd2) can be neutralized by 19...Qxd4. If I'm mistaken and White can gain an advantage even there, then this was an especially useful loss for Kramnik.
You've raised another interesting issue en passant: what is Kramnik going to play against Anand? If we are to judge by this tournament, Kramnik will use the Gruenfeld in the event that Anand plays 1.d4, but that seems highly unlikely to me. My inclination is to think Kramnik is largely hiding his repertoire, but rather than engage in Vizzini-like speculation ("It would be crazy to reveal my openings here. On the other hand, Anand knows that I wouldn't do it, so that's what I'll do! But he has probably thought of that too, so...") I'll just wait and see. If there is a good way to puzzle this out, the best means is probably to think about what openings would be most effective against the world champion while fitting with Kramnik's own tastes.