How do I calculate the "Total 3-year demand", "Total 3 year deseasonalized demand" and the "average 3 year deseasonalized demand"?

B C D E F G Month Demand Yearly average Seasonal Index Average SI Deseasonalized Time period 1/1/19 2192 0.7951 0.718 3051.14 2/1/19 2369 0.8593 0.811 2920.59 3/1/19 2462 0.8930 0.827 2977.89 4/1/19 2770 1.0047 0.927 2987.40 5/1/19 3281 1.1901 1.086 3021.57 6/1/19 3114 1.1295 1.075 2897.45 7/1/19 937 0.3399 0.636 1473.61 8/1/19 2315 0.8397 0.882 2624.48 9/1/19 2814 1.0207 0.948 2968.80 10/1/19 3318 1.2035 1.197 2770.87 11/1/19 3230 Put yearly average in below cell 1.1716 1.258 2568.03 11 12/1/19 4282 2757 1.5531 1.635 2619.22 12 1/1/20 2230 0.7257 0.718 3104.03 13 2/1/20 2394 0.7791 0.811 2951.41 14 3/1/20 2441 0.7944 0.827 2952.49 4/1/20 15 2701 0.8790 0.927 2912.98 16 5/1/20 3338 1.0863 1.086 3074.06 17 6/1/20 3232 1.0518 1.075 3007.25 2351 18 0.7651 0.636 3697.40 19 8/1/20 2806 0.9131 0.882 3181.12 20 9/1/20 2814 0.9157 0.948 2968.80 10/1/20 21 3592 1.1689 1.197 2999.69 11/1/20 22 4112 Put yearly average in below cell 1.3381 1.258 3269.27 12/1/20 23 4864 3073 1.5829 1.635 2975.22 24 1/1/21 2355 0.6345 2/1/21 ).718 3278.02 25 2951 0.7951 0.811 3638.10 26 3/1/21 2943 0.7929 0.827 3559.68 27 4/1/21 3333 0.8980 0.927 5/1/21 3594.58 28 3642 0.981 1.086 3354.03 29 6/1/21 3871 1.0430 1.075 7/1/21 3601.81 30 2979 0.8026 33 0.636 8/1/21 1685.05 3316 0.8934 31 0,882 3759.30 9/1/21 3367 32 0.9072 0.948 10/1/21 3552.22 33 4528 1.2200 1.197 11/1/21 3781.35 4690 Put yearly average in below cell 34 1.2636 1.258 3728.81 12/1/21 6564 35 3712 1.7685 1.635 4015.08 38 36 De 39 37 Highest yearly average 40 38 3712 year 2021 39 Total 3 year deseasonalized 41 Total 3 year demand demand 42 40 41 Average 3 year deseasonalized Average 3 year de-find demand 3180.5 42 43 44 45 46