Is sum of two uniform random variables is uniformly distributed? For example, $X_1, X_2 \sim U[0,t]$. Does it imply that $2X_1+X_2 \sim U[0,3t]$?

If $X_2$ is always equal to $X_1,$ and $X_1\sim\operatorname{Uniform}[0,t],$ then $2X_1+X_2\sim\operatorname{Uniform}[0,3t].$ At the opposite extreme, if $X_1,X_2\sim\operatorname{Uniform}[0,t]$ and $X_1,X_2$ are independent, then $2X_1+X_2$ is not uniformly distributed. You haven't told use the joint distribution of $X_1,X_2,$ but only how each one separately is distributed.
