Block Diagram for a difference equation
That's one way to do it but but typically you would have a single line of three delays in the feedback path and apply $a$ after the first delay, $b$ after the second and $c$ after the third.

And yes, you need arrowheads to make this clear.

By convention the coefficient of the feedback path would be called $a_1$, $a_2$, $a_3$ and the feed forward path would be $b_0$, $b_1$, etc. 

See for example https://www.researchgate.net/figure/Direct-Form-I-structure-of-IIR-filter_fig1_236688224