I am a K3 visa holder and I know this is a multi entry visa, my question is do they take my I-94 out of my passport at the airport or do i have to make sure it stays in there. And on re-entry to the US should i bring anything with me, or seeing my visa in my passport be enough?
Thanks.

The airline employees should take the I-94 from you, either at the check in desk or at the gate. I believe you should take NOAs for the K-3 with you as well, although often the visa is enough, on occassion of entering the USA on a visa I have been asked for the supporting paperwork. That was for H-1B however.