Code by @Thoralf-M

iota_multisig_demo.js

let multisig = require('@iota/multisig');
let converter = require('@iota/converter');
let txconverter = require('@iota/transaction-converter');
let bundle_validator = require('@iota/bundle-validator');

let seedA = 'A'.repeat(81)
let seedB = 'B'.repeat(81)
let seedC = 'C'.repeat(81)


let digest = multisig.getDigest(seedA, 0, 3)
let address = multisig.getAddress([digest])
console.log(address);

let digest2 = multisig.getDigest(seedB, 0, 3)
let address2 = multisig.getAddress([digest2])
console.log(address2);

let digest3 = multisig.getDigest(seedC, 0, 3)

let multisigaddress = multisig.getAddress([digest, digest2, digest3])
console.log(multisigaddress);

let MultisigInput = {
  address: multisigaddress,
  balance: 2,
  securitySum: 9
}
let transfers = [
  {address:'DHSZPMEJHCZNOVZAIGZRUOHAVDZUZXZLUNAEHYWG9RREOIO9PXFTXFGZDAJKDPRSKKEOME9PEOCI99999',
  value: 2}
]
let initBundle = multisig.createBundle(MultisigInput, transfers)


let firstSigned = multisig.addSignature(initBundle, multisigaddress, multisig.getKey(seedA, 0, 3))
let firstSignedTrits = new Int8Array(converter.trits(firstSigned.join('')))

let secondSigned = multisig.addSignature(firstSignedTrits, multisigaddress, multisig.getKey(seedB, 0, 3))
let secondSignedTrits = new Int8Array(converter.trits(secondSigned.join('')))

let thirdsigned = multisig.addSignature(secondSignedTrits, multisigaddress, multisig.getKey(seedC, 0, 3))

let finaltxobjs = thirdsigned.map(tx => txconverter.asTransactionObject(tx))
console.log(finaltxobjs);
console.log(bundle_validator.default(finaltxobjs));