get those boards off the floor
Any freshly milled boards will tend to warp if not stored so that air can circulate freely. I also store some with binding tape on them if they are thinner. You can clamp them together until you are ready to use them.
I see a radiator of some sort, hopefully not steam which would heat the room unevenly. Maybe it's not in use yet.
Avoid direct sunlight also. Flip them over so they are exposed evenly.
Finally, I would not use biscuits as alignment aids. They can be a pain if the slots are not equal distance from the "show" face and will actually induce a misalignment. Build up the table in 3 board sections, them glue the sections together. You can control for flatness/alignment easier this way.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)