Creating a multi-sig wallet in Electrum is not simply a case of combining 3 standard xprvs or xpubs. You have to specifically create a multi-sig setup.
Initialize 3 different ledgers, as legacy. Copy the master public key seen with Wallet->Information for each of them, with the form xpub.
This is where you are going wrong. You can't just mash together 3 standard xpubs and create a functioning multi-sig wallet.
You need to create a new wallet, select "Multi-signature wallet", select 2 signatures from 3 co-signers, select "Create a new seed", choose Legacy or SegWit, back up your seed, and then it will give you a multi-sig xpub which you can use. Repeat that process twice more to get your 2 other xpubs.
You can then create a watch-only multi-sig wallet by combining the three xpubs, and the three standard single signatory multi-sig wallets by combining one of the seed phrase with the xpubs derived from the two other seed phrases. You can even create a single wallet capable of signing transactions itself by combining two or even all three seed phrases in to the same wallet, although that obviously defeats the purpose of a multi-sig wallet.