"Multisig P2P" sounds complicated.
What it means, specifically, is that HodlHodl can help if there is a dispute.
Having one of the three keys lets them either:
  • Agree with the Buyer (and release the seller's escrowed coins to the buyer), or
  • Agree with the Seller (and permit the seller's escrowed coins to return to the seller)
This way, HodlHodl doesn't ever have control of your coins in your wallet. For a transaction in progress HodlHodl has limited power (as described above) and nothing more.