I want to verify that I have well written the three seeds.
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 ?
By following that method, just make two copies of the PSBT with 1 signature and send it to the other two cosigners.
Either one can sign it.
But this made me think that your 2-of-3 MultiSig wallet actually have two seed phrases and one master public key.
In that case, create a new 2-of-3 MultiSig wallet with the third seed phrase and the other two's master public key, see if it can restore the correct addresses.
You can send the PSBT file as well for your peace of mind.