In order to specify a 2d vector field $\mathbf{F}(x,y)$, we need to find the $\mathbf{i}$ and $\mathbf{j}$ components of $\mathbf{F}$ at all positions $(x,y)$.
To specify a 3d vector field $\mathbf{F}(x,y,z)$, we need to find the $\mathbf{i}, \mathbf{j}, $ and $\mathbf{k}$ components of $\mathbf{F}$ at all positions $(x,y,z)$.