What is the characteristic sign seen in this case?
Twin peak sign or lambda sign is seen, which suggests dichorionicity.
What is its equivalent in triplet pregnancy?
Ipsilon zone thickness
Is it an absolute sign, which suggests dizygosity?
Absolute sign is demonstration of different sexes. However, twin peak sign is pretty suggestive in early gestation.
The placenta is seen insinuating between the layers of the membrane at its edges (twin peak sign), a finding that is suggestive of a dichorionic gestation.