Regarding mathematical statistics, solve $($a$)$ and $($b$)$

Assume that X$1,$ X$2,$ X$3$ Exp$(\backslash$lambda $)$ are independent and equally distributed with distribution Exp$(\backslash$lambda $=1).$

$($a$)$ Determine the distribution of Y$=$X$1+$ X$2+$ X$3$ and state its PDF f $($y$).$

$($b$)$ Determine the distribution of U $=2$Y