Natalie, it's so brilliant that your sister chose a colour that you look stellar in! I'd say keep all three of these dresses for one occasion or the other, if you can --- wow! Though I agree, the third is best as a day dress, probably, rather than at a wedding.
I totally see the charm of #1, and like many others, was leaning towards it from the thumbnnails alone. But then I saw #2...! (Are you hearing 'broken record' yet?) I love the way it shows off your pretty shoulders and flatters your bust, and it's so ladylike. I understand the 'maternity silhouette' issue with the empire waistband viewed in profile, but I wonder if a more supportive foundation to lift the bust wouldn't result in a more curvy shape. And if you're going to be moving around a lot, and especially if you're dancing, now only will you be more comfortable in #2, you'll be showing off your silhouette beautifully with that swirly skirt.
For photo ops, I'd suggest facing the camera or using your purse or clutch (assuming you'll carry one?) to, uh, 'dent' the skirt under the bump when side-on to the camera --- I'm not explaining this very well, sorry.
You're going to be glowing, either way!