To create a multisig session: 
const session: Session = {
  owners: {
    type: 'ecdsa',
    accounts: [accountA, accountB, accountC],
  },
  policies: [
    {
      type: 'sudo',
    },
  ],
}
You can also specify the signature threshold (i.e., how many signatures are required to authorize the transaction): 
const session: Session = {
  owners: {
    type: 'ecdsa',
    accounts: [accountA, accountB, accountC],
    threshold: 2,
  },
  policies: [
    {
      type: 'sudo',
    },
  ],
}
By default, the threshold is 1 (any session owner account can authorize any transaction within the session).