Flankers were NOT designed to carry drop tanks. Su27, Su30, Su33 did not feature any plumbing to use drop tanks. Su33 does have the plumbing for the centerline buddy refueling pod though. So in theory, it could require least testing and integration effort to have a centerline drop tank retrofitted to it.
Su-35 was initially advertised as being design to use drop tanks. (2000 L tanks) BUT to date there has never been a single image published with Su35 actually carrying drop tanks. In all likelihood, such tests never happened. Now, does that mean that Su-35 has the plumbing but the integration tests were never performed OR that the requirement altogether was deleted and that Su-35 didn't even get the plumbing - we don't know. No way of knowing.
Su-34 is the only member of the russian flanker family that was designed for tanks, was tested with them and carries them operationally, when needed. Again, allegedly the underwing points were designed but to date there's never been an image of su-34 with underwing tanks. (allegedly 2000 L ones) But, what has been seen is the large centerline tank, a 3000L one. Its use is quite normal for Su-34.