From $2003$ through $2008,$ the sales $R_1($in thousands of dollars$)$ for one of two restaurants owned by the same parent company can be modeled by

$R_1=480-6t-0\mathrm{.}6t^2,t=3,4,5,6,7,8$

where $t=3$ represents $2003.$ During the same six$-$year period, the sales $R_2($in thousands of dollars$)$ for the second restaurant can be modeled by

$R_2=259+0\mathrm{.}77t,t=3,4,5,6,7,8$

Write a function $R_3$ that represents the total sales of the two restaurants owned by the same parent company.