Answer 1

Three elements are used after # as shorthand where each element is doubled.

e.g
#123 = #112233
#000 = #000000
#fff = #ffffff

Answer 2

It's a common shortcut for colors with all the same characters

# introduces the color hexadecimal and three characters are compiled as six...

#FFFFFF is white

Answer 3

rgb(256,256,256) is The last possible values ​​are for coloring,
and in hexadecimal value last value is f

#fff=#ffffff is white because its last of value.