These effects accumulate, so if you have 3 “Harpie Lady” on the field all 3 effects are applied. Only face-up Harpies count, though.
If you activate the 3rd effect of “Harpie’s Pet Baby Dragon” and the opponent chains an effect to destroy 1 of your other “Harpie” monsters, leaving you with only 2, the effect still resolves properly because the conditions were correct when it was activated.