  We study the melting of three-stranded DNA by using the real-space
renormalization group and exact recursion relations. The prediction of an
unusual Efimov-analog three-chain bound state, that appears at the critical
melting of two-chain DNA, is corroborated by the zeros of the partition
function. The distribution of the zeros has been studied in detail for various
situations. We show that the Efimov DNA can occur even if the three-chain (i.
e., three-monomer) interaction is repulsive in nature. In higher dimensions, a
striking result that emerged in this repulsive zone is a continuous transition
from the critical state to the Efimov DNA.
