One way is to define colorbar=false in all 3 subplots and then use @layout to add the single colorbar:
p4 = plot(p3, framestyle=:none, colorbar=true, title="", lims=(-2,-2); config...); # empty plot
layout = @layout [[a b; c] d{0.05w}]
plot(p1, p2, p3, p4; layout, dpi=600, size=(680,600))
