Had three electrons been possible to be placed in an orbital, the block of potassium in the periodic table would be:

The electronic configuration for this case can be given as,

$1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2,3{\mathrm{p}}^6,4{\mathrm{s}}^1$

This electronic configuration considering given conditions can be given as,

$1s^3,2s^3,3p^9,3s^3,3p^1$
So, the block of potassium in the periodic table would be p.