Because the maximum multiple of 3 less than 19 is 18, Xiaodong eats 1 capsule first.

Then, no matter how many pills Xiaohua takes, Xiaodong takes the number that adds up to 3 with Xiaohua.

That is, Xiao Hua Na 1 and Xiao Dong Na 2.

Xiao Hua takes 2, Xiao Dong takes 1.

Finally, it must be small.