UPDATE 2: To put OP out of their misery find now at the very bottom of this post an answer to what they probably mean.
UPDATE: OP has changed the rules, so this is no longer valid, but see bottom of this post for an answer to the modified question which I assume is still not what OP has in mind.
I say it is
With the new rules the answer is
If we require tiles to be on grid then the answer is
We have to cover at least one square of each 3x1 subgrid, in particular
the four shaded subgrids. No tile can overlap with more than one of those. So we need at least one for each shaded subgrid.
We now show that we need one additional tile. Indeed, we also need to cover each row and each column of the central 3x3 subgrid. As the 4 so far allocated can each cover at most one row and two columns or one column and two rows, at least two of them must "point inside" (not straddle the edge of the table). At least one must therefore be off-centre:
On the larger (right in example) side we have another 3x3 subgrid which cannot be covered without adding a fifth tile.
Almost forgot (thanks @justhalf):
5 is sufficient: