Rank the following substituent groups in order of decreasing priority according to the Cahn-Ingold-Prelog system :

$\underset{1}{-\mathrm{CH}{\left({\mathrm{CH}}_3\right)}_2}-\underset{2}{{\mathrm{CH}}_2\mathrm{Br}}-\underset{3}{{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}}$