Assuming you have 2-of-3 multisig wallet and you have the 3 seed phrases in different locations. It is kind of confusing me because to me it is still 2-of-3 multisig wallet in which only two seeds are need to make transaction, not three seeds. And also this setup to me is still 2-of-3, not 3-of-3.
The wallet itself is still 2-of-3, but if you have neither the seed phrase nor the master public key from the third share, then two seed phrases alone cannot regenerate your wallet and so cannot spend the coins. If you only back up three seed phrases and lose one of them, your coins are lost.You can have digital copy of the pub keys. Anyone who have access of all the Master Pubic key does not mean they can restore the main wallet. As per my understanding it will restore a watch only wallet. With a watch only wallet they can not move the coin so you are safe.
You are correct in saying all the master public keys only allow the creation of watch only wallet which cannot spend your coins, but such a wallet also completely compromises your privacy and makes you a target for the attacker who was able to compromise your master public keys to now attempt to compromise your seed phrases.I can write the 3 seeds on a paper with a pen but what about the pub keys 
Too many characters and a huge risk of mistyping.
You have three options, ranked in order of security from best to worst:Too many characters and a huge risk of mistyping.
1) Write them by hand carefully, double check your work, and then test that you can recreate the wallet from what you have written. If you can, then you can rest assured you have not made any mistakes.
2) Print them using a dumb printer.
3) Save them electronically.