unsigned char temp_array[3]={10.5, 40.5, 30.6};

an unsigned char is an 8 bit whole number 0 to 255 aka an integer.
"C" chops of the decimal for you.

float temp_array[3]={10.5, 40.5, 30.6}; will do as you ask.
BUT, floats eat up RAM and ROM and CPU cycles very fast in 8 bit CPUs. Avoid them if any of that concerns you.

Example use:

unsigned int temp_array[3]={105, 405, 306};
unsigned int temp;

and

temp = getHighest(temp_array,sizeof(temp_array));
printf("%5d.%02d \n",temp ,temp );

Also note that printf() is also a large piece of code.