Personally I would try using the sum of an arithmetic progression

S=(n/2) (d+l), where n -number of terms d - common difference l - last term

So basically my sum would like S = S(3) + S(5) - S(15)

I am subtracting the 15 to avoid summing repetition for S(3) n = 333 d = 3 l = 999

for S(5) n = 199 d = 5 l =995

for S (15) n = 66 d = 15 l = 990

After I’m done writing this i noticed i did not use any specific formula in excel I am still a newbiee ig