We consider a dense urban cellular network where the base stations (BSs) are stacked vertically as well as extending infinitely in the horizontal plane, resulting in a greater than two dimensional (2D) deployment. Using a dual-slope path loss model that is well supported empirically, we extend recent 2D coverage probability and potential throughput results to 3 dimensions. We prove that the "critical close-in path loss exponent" $α_0$ where SINR eventually decays to zero is equal to the dimensionality $d$, i.e. $α_0 \leq 3$ results in an eventual SINR of 0 in a 3D network. We also show that the potential (i.e. best case) aggregate throughput decays to zero for $α_0 < d/2$. Both of these scaling results also hold for the more realistic case that we term ${3\rm{D}^{+}}$, where there are no BSs below the user, as in a dense urban network with the user on or near the ground.