Is watching the first 2 seasons of the anime enough to understand "Tokyo Ghoul:re" manga without reading prior manga? I've watched Tokyo Ghoul and Tokyo Ghoul Root A, but I didn't read the manga.

Is watching the them enough to understand Tokyo Ghoul:re manga without reading prior manga?
One big thing to note is that Tokyo Ghoul Root A (the second season of the anime) greatly diverged from the manga. In the manga, Kaneki left Anteku and went off on his own, forming his own group. In the anime, he joined Aogiri. 

Tokyo Ghoul:re is a sequel manga, meaning it builds off of Tokyo Ghoul. The first season of the anime does only a decent job at covering the story of Tokyo Ghoul, and the second season, as noted, diverged, covering only a handful of the biggest plot points. Having only watched the anime, you will not know numerous plot points that are addressed or expanded upon in :re. 

However, in my personal opinion, you will only be moderately confused reading :re if you only watched the anime. Thankfully, Root A converged with the canon story line near the end, and had many of the major events occur identically to the manga (albeit several of them in the wrong order, and as such events that drove them to the order they were in are skipped in the anime). So as an anime only viewer, you would have knowledge of almost all of the major plot points.

With another however here, I can not find one person who would recommend doing so. Almost everyone who claims to be a fan of Tokyo Ghoul the manga greatly dislikes the anime. Even the top reddit post asking where season 2 starts in the manga is met with universal replies of chapter 1 (as in, pretend you never watched the anime and start the series from the beginning). As noted in Why did Tokyo Ghoul √A completely deviate from the manga?, the differences in season one are fairly significant, and you are going to be hard pressed to find people who prefer the anime's story over the manga's.

If you are desperate though, and refuse to read from chapter 1, then season 2 starts at around chapter 46.