There are a lot of ways to look at three in a bed, both from husband and wives point of view, but physical logistics is an issue right up front and I'm talking about sleeping, not the other stuff. Most folks like to sleep within touching distance but not touching, which requires space.
Depending on the size of the folks involved, a King-sized bed may not work in the beginning and probably won't work over time. Even if everyone is comfortable, with one pregnant wife something needs to change. With two wives pregnant, something *has* to change. Someone has to sleep in the middle (probably won't be him- part of his job is investigating noises in the night) and getting in and out bed during the night without waking the others up can be difficult if not impossible if the bed is too small. Add in any discomfort involved while getting used to having more than one wife in bed and bigger is most definitely better.
The best solution is to build the bed, not buy one. If he's not handy with tools, he should hire someone who is. A custom furniture maker is ideal. If you think about it, 3 FAA "standard persons" come in at 170 lbs each, which is 510 pounds. Add the weight of the box springs and mattress and now we're around 750 pounds. Did I mention the dynamic load stresses caused by both horizontal and vertical movement that may involve all 510 pounds at once? No? Let's just say it adds to the stresses the bed needs to support across a greater span. That's engineering talk for "it had better be sturdy or one day you'll tell the story about 'the night we broke the bed' to your grandchildren." Or maybe you'll just suppress the memory of waking up the entire house under... odd conditions.
Pay attention to the design because this is going to be with you for a while. Some folks like beds that are designed with posts and an overhead so if you want, a canopy can be hung and it's like a tent. That's handy if you have kids who pop in out of nowhere at the worst possible time. However, the footboard needs to be low enough that whoever is sleeping in the middle can get in and out of the bed that way. A cedar chest for the linens at the foot of the bed can be helpful in that regard, especially in the bed is high. Logically you'd want the bed low, but most don't want to give up all that floorspace in terms of at least *some* storage under the bed because a bed 9-10 feet wide is huge and eats up a lot of bedroom space.
Any good hardwoods like Oak or Maple should work out well. You'll also find it works better if the legs have bases at least 6"x6" to help spread the weight out. That also makes it easier to cleat the feet to the floor so the bed *won't* move (our 3 "standard persons" all moving around at the same time places a lot of lateral stress on the bed and it *wants* to move to relieve those stresses). You may want to consider putting rails on 2' centers from the headboard to the footboard and bolting them on with oversized wingnuts so they can be tightened from time to time. Nobody likes a squeeky bed, especially if teenage kids are in the house (you'd be surprised how sensitive cell phone microphones are).
Then comes mattresses and you really don't want to Google the cost of oversized mattresses. Just don't. DIY in this area isn't difficult and what you want are the new-generation foam mattresses, which can be combined without a lumpy "seam" running down the middle of the bed. For 3 people, two full mattresses (full XL if tall people) should work, that's 9 feet across. Figure 6 inches in from each side because nobody likes laying too close to the edge and that's 8 feet of mattress space for 3 people. The nice thing is they're double-sided so they can be flipped whenever you want- it just takes 2 people.
(If you use queen-sized mattresses- 10 feet wide -you can use 3 twin-sized box springs underneath for more support and less money)
Top off the joined mattress with a foam "egg-crate" mattress topper. Those are reasonable to have custom made and if you use foam mattresses and the foam topper you'll never know where the seam is. Next comes sheets and prepare to either get handy sewing or be ready to write large checks because oversized custom sheets, comforters and duvet covers are not cheap. There are a number of folks who make them and a set of sheets, pillowcases, sham, duvet and comforter will start around $500 and go from there. The saving grace to this is since they're custom made, you can get exactly what you want. 600 thread-count on all the sheets and pillowcases? No problem, just send more money. Does somebody have a habit of stealing the covers? Make them bigger.
One word of warning, for your sleeping arrangements a good policy is "Nobody's Business But Ours" (NBBO) for a lot of reasons. That includes the kids because they will be asked, but I've found that women in the church are the worst when it comes to prying. Visitors in the home should be considered because if seen, a bed that size will raise as many questions as it answers and become a subject of gossip in and of itself. How you choose to respond is up to you but if you're known to be poly then people will talk and there should be a family policy everyone abides by. My advice is to frame it in terms of equality- "We're just like you, we didn't get married to sleep alone... but what happens between us is nobody's business but ours... just like in your marriage."
That's the how. As to the why, there are some really good reasons for everyone sharing a bed that have nothing to do with sex, but sex is always in the background. In fact, if you look closely at Leviticus 18:17-18, the presumption of those statutes is the wives would "uncover the nakedness" of each other in the process of sharing a sexually active bed. It's unavoidable at one level or another. In other words, outside the prohibitions it's not immoral no matter what happens but it's NBBO.
