LedgerJS - How to P2SH sign multiple transactions from multiple wallets in 1 function call?
The third parameter is just your outputs script (actually it's the whole outputs part of a transaction) so depending on which address you want to send the funds it will change.

It's using the same format as in bitcoin transactions. The data are serialized like this

TYPE | Description
VARINT   | Number of outputs to create
8 byte   | Amount to send to the first output 
VARINT   | Size of the output script
x bytes  | The actual output script

In most case you can consider that VARINT is only 1 byte (special cases if you data is bigger than 255 bytes but if that's the case you probably have an issue somewhere :P)

Regarding the output script depending on which address you want to send you have different type of scripts to use.

btc.serializeTransactionOutputs(???).toString('hex') is just serializing the outputs part of the transaction from a split transaction. If you use it on an existing transaction you're basically sending to the same addresses (the same amounts to the same addresses).