According to slaters rule order of effective nuclear charge for last electron in case of Li, Na and K.

Effective nuclear charge Z_eff = Nuclear charge (Z) - Screening constant ($\mathrm{\sigma }$)

Formula used to calculate '$\mathrm{\sigma }$' =(0.85×n_p+2×n_up)

where n_p is number of paired electrons and  n_up  is number of unpaired electrons 

( I ) Li(Z=3)...1s²2s¹
σ for lithium = 2(0.85)=1.7
Z_eff = (3-1.7) = 1.3

( II ) Na(z=1)...1s²2s²2p⁶3s¹
σ = 2(1)+8(0.85)=8.8
Z_eff = (11-8.8) = 2.2

( III ) K(z=19)...1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
σ = 10(1)+8(0.85)=16.8
Z_eff = (19-16.8) = 2.2