Hello guys! I just read an article that says addresses start with 3 don't have signed message functionality. Can anybody help me with this? Thanks
No, that's an article's mistake or you misread it. You can sign messages on each kind of address. Whether it's a P2PKH, also known as legacy, (1...) or a P2SH (3...) or a Bech32 (bc1...).
Have you tried
electrum? It's an open source wallet that can do literally anything compared to some other wallets, such as the online ones.