What exactly is this segwit address I have starting with 3, and why's there no privatekey to it?
Read the thread, I already explained above.
What happens if I send funds to it? Is the balance held under the orig address?
No, it is held under the '3' address.
Addresses and public-private key pairs are different things. A public-private key pair can be associated with many different addresses, not just one '1' address.
What's the difference between this and the first non-segwit address?
It uses segwit.
Why should I use 1 over the other?
Segwit has lower transaction fees.