Out of curiosity, why would you want all 3000+ topics to have the same date?
But, curiosity aside, you should be able to do this with the API or the rails console, if you’re comfortable using it (it’s good practice to take a backup first, just in case).
There’s some info about how to get the relevant API for things in this topic - Reverse engineer the Discourse API
And there are a few examples of different rails techniques in Administrative Bulk Operations that you could use to build what you need (or give you some idea of what to Search for).