depending on your budget i would get perhaps get the twins a combined present and the little one something practical
saw an outdoor thing for the twins- a see saw ? totem tennis set? hopscotch and skipping ropes? even some scooters- you can pick them up for $30 each!!

for the little one- perhaps some cute outfits? even a little trike? or ride on little push car thingy

definately not more indoor toys as i am betting that they have enough to sink a battleship with 3 girls!!

other things that they all could share is an outdoor table and swing seat, portable DVD player for the car, even just a bunch of DVD's!!!

then if you get desparate there is always the game theme too use as well!!! good ole family fun games!!
HTH