Function f is defined for every $\mathrm { x } \in ( 0,3 ]$ as
$$f ( x ) = 2 x + 1$$
and satisfies the equality
$$f ( x ) = f ( x + 3 )$$
for every real number x.\\
Accordingly, what is the sum $\mathbf { f } ( \mathbf { 6 } ) + \mathbf { f } ( \mathbf { 7 } ) + \mathbf { f } ( \mathbf { 8 } )$?\\
A) 8\\
B) 12\\
C) 15\\
D) 18\\
E) 21