You asked better way to copy the first two elements from the array1 and three elements from array2 and stored into array3.Following example better way for your requirement.
If you use this program you don't want to manually copy the elements from the array.
Code:
#include <stdio.h>
#include <string.h>
int main ()
{
int i;
double array1[3] = {1.6,1.3,1.9};
double array2[5] = {2.1,3.6,4.6,8.2,9.9};
double array3[5];
memcpy (array3,array1,2*sizeof(double));
memcpy (array3+2,array2,3*sizeof(double));
for(i=0;i<5;i++)
{
printf ("array3 : %.1f\n",array3[i]);
}
}