Recall that a *double-stochastic matrix* is a square matrix with non-negative elements such the sum of the elements in every row, as well as in every column, is $1$. The set of all double-stochastic matrices is described by the [Birkhoff – von Neumann theorem][1]. For the matrices of prime order $p\ge 3$, this theorem can be interpreted to describe the set of all functions $w\colon\mathbb F_p^2\to\mathbb R^+$ such that for some pair of fixed directions in $\mathbb F_p^2$, every line $l$ in any of these directions gets its exact share of the total mass of $w$:
  $$ \sum_{x\in l} w(x) = \frac1p\,\sum_{x\in\mathbb F_p^2 } w(x). \tag{$*$} $$

Let's say that a line $l\in\mathbb F_p^2$ is even if it satisfies ($*$), and that a direction in $\mathbb F_p^2$ is even if each of the $p$ lines in this directions is even.
Suppose that, instead of just two even directions (as in the Birkhoff – von Neumann theorem), there are $k\ge 3$ even directions. Intuitively, one can expect much more structure in this case. 

> For a prime $p$ and integer $3\le k\le p$, what is the set of all functions  $w\colon\mathbb F_p^2\to\mathbb R^+$ possessing at least $k$ even directions? 

(A word of caution, in case you are about to google for "triple-stochastic": this term is normally used for a three-dimensional array with the sum of the elements equal to $1$ along each of the three "coordinate directions".)


[1]:https://en.wikipedia.org/wiki/Doubly_stochastic_matrix