Address starting with 3 is a multisignature address, a address requiring 2-of-3 signatures to be able to be spent. It is an address associated with more than one ECDSA private key. It is usually used as cold storage as it is harder for one to steal 2 private keys compared to 1. It can also be a protection against malware as you could keep the keys on separate computers. Keeping one of the three keys safety is also a good practice, if one of the keys is lost, the bitcoin can still be spent using 2 of the remaining keys. It can also be shared between multiple people, majority of the people can vote and spend the coin.