A3B, C8E, ______, G18K, I23N, K28Q

I group of  letters follows +2.  Next letter is $\mathrm{C}+2=\mathrm{E}$

II group of  Number follows +5.  Next number is $8+5=13$

 III group of  letters follows +3.  Next letter is $\mathrm{E}+3=H$

 So, the  answer is E 13 H.