Moonsea might pull the biggest population card, but don't expect any of the US servers to have a big population advantage. LOTR did their 64b server merge in March of this year. Go read their forums if you like. The players had the idea to create super-servers, and 2 worlds became the favorite target destination. When transfers started and the devs spotted those worlds getting more populated than the others they shut down transfers to those worlds, forcing people to pick someplace else.
Sev has already stated they intend to keep the 3 US DDO servers balanced. They already went through this on LOTR, they do read the forums, they know what to watch for. If Thrane starts pulling more than it's share they will make adjustments.
I think 3 servers is overkill, 2 US servers would have been more than enough. You have to go back to May 20, 2023 to find a peak population of more than 3600. 2025 hasn't broken 3000 players online yet, and on average only has around 1800 players across all words at any given time. Take out the Moonsea population, split what's left over 3 servers ...
The increase in populations will be nice, but don't get your hopes up too high.