with a singlesig wallet, I can check my balance on the internet thanks to the pub key.
Will it be the same with a multisig as there will be 3 pub keys instead one ?
By saying "
internet", do you mean via blockexplorer?
And by saying "
balance", do you mean the entire wallet's balance?
If so, most of them do not accept multiple master public keys to check for transactions of your MultiSig Electrum wallet.
It'll only work on standard wallet's master public key.
Otherwise, just paste your MultiSig address and it'll search just like in SingleSig addresses.