\noindent{\bf Ans}:
Relative error $ = \left| {e_n \over p_n^{\rm exact}} \right |$.
 ({\bf 3 pts})

Note that
$$
e_n \approx 
= d_1 1^n + d_2 2^n + d_3 ( {1\over 3} )^n \qquad {\bf (3 pts)}
$$
$d_1$, $d_2$ and $d_3$ are of $O(p_i - fl(p_i))$, $i= 0,1,2$.
Therefore 
$d_1$, $d_2$ and $d_3$ are of $O(\delta)$.{\bf (1 pts)}.

Therefore, relative error
$ = 
{O(\delta) 1^n + O(\delta) 2^n + O(\delta) ( {1\over 3} )^n
\over
c_1 1^n + c_2 2^n + c_3 ( {1\over 3} )^n
}.
$
It is
stable if and only if $c_2 \neq 0$.
({\bf 3 pts})