Let the work to be done is $12$ units.(Take any number that is divisible by $4,3,2$ like $12,24,36….$ ).
Now since $A$ can do it in $4$ hrs, $A$ completes $3$ units of work per hour.--->$(1)$
$B$ and $C$ can do it $3$ hrs.So together they complete $4$ units of work per hour.--->$(2)$
$C$ and $A$ can do it in $2$ hrs.So together they complete $6$ units of work per hour.--->$(3)$
from statements $(1),(3)$ we can say that A does $3$ units of work per hour.--->$(4)$
from statements $(2),(4)$ we can say that B does 1 unit of work per hour.
So to complete $12$ units of work, $B$ need $12$ hours.
The $12$ units of work can be anything it can be building $12$ walls, eating $12$ apples,manufacturing $12$ computer etc….