Do you have a problem with the 3 year old getting out of bed? If so, having her upstairs shouldn't be that much of a concern. When my oldest 2 were babies, we were all upstairs (master up), but I still worried about my kids getting out of bed and trying to come to me and having to pass the stairs on the way. I used 2 methods depending on their age and my own preferences, but I put a child knob on the inside of their room. This helped for time outs, and if they woke at night, they couldn't open the door. I had a baby monitor and could hear if they called for me, so I pushed that with them "if you ever need mommy in the night, just yell for mommy and I'll come". And I did go to them if they called. The other option was a gate at the top of the stairs. We put it up at night for the same reason, if they wandered out of their room, they couldn't fall down the stairs.
If it were me, I'd either move the 3 year old upstairs, or everyone. If you just move the 3 year old, then you make it a big deal "guess what! You are such a big girl now that you get to get a big girl room UPSTAIRS!!! Isn't that cool! How about if you pick which room you want upstairs and we'll talk about making it yours?" If you can let her pick between some rooms upstairs, that would be great. At 3, you should be able to persuade her to pick the room that you want her to have. Tell her she can pick her room color (within reason) and have the room painted so that she is excited about the move. Don't move the twins into her room downstairs right away when she moves out, give it a month or so, then move them in. Or, again, I'd say put the twins upstairs too, but that's climbing stairs in the middle of the night. Finally, just because their rooms are upstairs doesn't mean they'll play up there, maybe that is your husband's concern? My kids only played in their rooms when they were little and their rooms were upstairs if I was also upstairs - if I was cleaning or putting away clothes, if not, they were downstairs and we just kept a basket full of toys and some books downstairs to play with. Oh, another idea for room safety while they are upstairs is to put a gate on their door. Meaning if you want their door open and maybe you can't get a gate on your stairs, you can put a gate in their doorframe so that they can look out of their room, but they can't get out of their room. I used to do that while I was cleaning upstairs so the kids would stay in one room and play and the door was open so I could still see them easily.