I see where you are coming from. Imagining a pyramid shape, you're assuming that the negativity of the lone pair at the top cancels out the negativity caused by the electronegativity of the chlorines which are pointing down. This is not the case, the negativities are not equal, the lone pair or the 3 Cl atoms, one will be more negative than the other, so there will be a dipole (although I'm not completely sure in which direction).