Hello and welcome to the forum; though it’s a pity, that you come here for that reason.
Apart from the rice-thing, it seems, that you did it all right, though I don’t know, how long it really takes to get the water/moisture out of the least corner of the phone.
Unfortunately I am not really a tech guy.
Maybe you could try another display; if you know someone else with a FP3, that would be willing to assist. (Or - the other way around - you could try the display in another phone.)
That’s the first thing, I would do. If you don’t know someone with a FP3, the fairphoneangels might be an option. You will find a map with angels’ heavens, when you follow the link.
If it’s the core-module, this might be a total loss, as a new core-module has to be changed by support. The price for this repair, that they give on their homepage, is € 209.90.
That would be - of course - worth a repair in itself. But as the phone was submerged for a while, you might as well run into trouble with other modules as well; now or later on, that is.
You can contact support anyway to let the check the phone.
This will at least cost you shipping, handling and diagnostics costs; though it’s unclear, how much that would be. Or you get a repair. See Return Policy, point 4
I keep my fingers crossed, that you can solve your problem and revive your phone. 