A hostess gift is always nice but not necessary....an open house means come and go as you wish.....but don't stay for the whole time on the invitation.
The fact that there are 3 couples on one invitation indicates that it will be a BIG party...perhaps trying to fulfill all obligations at once.
It is always obligatory to respond to any invitation....RSVP or not....just good manners!
I would take a small gift to the people I knew....but not to the one with 3 people hosting....and remember to write a nice note to the 3 hosts.
Linda C

An open house is the kind of a party people usually have if they're inviting a lot of people -- too many to fit in the house all at once. The idea is for folks to come and go to sort of spread out the guest list over several hours, thus avoiding overcrowding and giving the host/hostess more opportunity to visit with more of their guests.

I would agree with everything Linda said. As for bringing food, I'd probably ask the hostess what I could bring. I just do this as sort of standard procedure to any party I'm invited to anyway. That way, if the party is being catered and the hostess has everything set just so, she can say don't bring anything.

On the other hand, you won't show up with no food to a party where everyone else brings something. I've had that happen to me -- everyone else knew to bring something since it was an annual party they had attended before, we weren't asked to and didn't know since it was our first time!

Wine or a seasonal flowering plant is nice when you're not sure about the food. I like to take bulbs like paperwhite narcissus or amaryllis just ready to flower. They're inexpensive enough, easy to dress up, you don't have to do any cooking and no one has to eat them!!

And ...re: the flowering bulbs....they bloom and die and you can throw them away!!
I like to give "disposables"...like wine, flowers, very nice paper napkins, candy, nuts etc.
Linda C