Is it enough to store the seed of the electrum or we must have to store the private key of all of the addresses too? I'm using electrum and only stored in seed in 3 physical places.
It is enough to keep only seed of Electrum wallet, but you should be 100% sure that seed is correct. When it comes to Electrum it is only 12 words, so that the possibility of a mistake is somewhat smaller then with hardware wallets seed which have 24 words. Sometimes people write words in some unusual order, mixing them in horizontal and vertical direction, which can cause big problems in case of recovery. Users should not
play with private keys in HD wallets, it's just another added risk, but in case you have all coins in one or two addresses printing such private key on paper may be additional insurance.
This doesn't make any sense. Blockchain.com gives you access to your private keys of the addresses and you have 100% access to everything.
Blockchain.com does not give users direct access to their private keys in a way that you can see private key of address just by click on address. If user for some reason need to get private key then only way is to use seed from this wallet and extract private keys with some tool (iancoleman). However if you import private key/s in this wallet (which is possible), then you have full control on that address / private key.