Project $3$ can be selected only if both Projects $1$ and $2$ are selected, but if both Projects $1$ and $2$ are selected, Project $3$ doesn't have to be selected. The appropriate constraint would be

$x3x1+x2$

$x1+x2x3$

$x1x3$ and $x2x3$

$x1+x2+x32$

None of the above