There are claims that the 3500 number
was first put out by TASS - Russian govvie news organization. Then withdrawn. Given the imagery of convoys destroyed, failed airborne attacks on airports, failed amphibious assault on Mariupol, I find the number plausible.
That said, Russia is still advancing and gaining ground, and I expect Ukrainian casualties might be just as high, if not higher. There were some crazy big rocket barrages in the east, some huge airstrikes there (near Kharkiv) in the beginning of all this.
Its still to early to call this a failure, or anywhere close to that. Russia is having a harder time than anticipated, but it is still only day three. For reference, it took the U.S. six weeks to recapture Falluja in Iraq, four weeks to recapture Hue after the Tet offensive.
It's really just a question of how many casualties Russia can tolerate - and they have a history of tolerating
insane casualties among their own troops.