Where are your xpubs backed up? They should be backed up alongside your seed phrases.
I have to create a "fake" transaction on electrum then sign it with the first key then export the PSBT then sign it with the second seed ? What about the third seed to check it's correct ?
What is contained by the wallet you plan to create this fake transaction on? All of the seed phrases? Or just the xpubs? Or some combination?
How are you planning to sign it with second seed phrase? Are you importing that seed phrase in to a clean device alongside the two other xpubs? Again, where are your xpubs backed up?
I would not create a transaction at all. What I would do instead would be to access each seed phrase back up individually, use each back up to derive the necessary xpub, then take all three xpubs together and recover a watch only copy of your multi-sig. If the addresses match up, then your seed phrases are correct.
I would also point out that creating a multi-sig on a single device negates the whole point of a multi-sig, which is to remove a single point of failure.