Bra sizing can definitely be tricky. Our specially trained bra fitters in store will be able to fit you and show you styles that will work best but if you can't make it in to a store we recommend determining cup size by how the bras you already own fit. Pay close attention to how the wire fits, the wire should lay flat in the center and encircle all of your breast tissue around to the side close to your underarm.
If DDD styles you're wearing currently fit that way then that's your best size. However, if say a 36 DDD fits well in the cup but the band is a little loose then you might want to try a 34 DDDD or G in US cup sizing because when you go up or down band size you have to adjust the cup size too.
If the wire on some styles feels like they're poking into you or you're spilling out of the cup you may want to go up a cup size. Your bras should be comfortable and supportive for all day wear and based on my experience as a certified bra fitter on our sales floors I'm sure there is a bra out there that will provide that for you.
With all that said, this particular style is wireless so the perfect wire fit isn't necessary and I would say if you normally wear a DDD, you're going to be fine ordering a DDD in this style. Just make sure the band will be nice and snug because that's where the support should come from, not the straps.
I hope this helps and if you have further questions, I'm happy to help here or I would try calling a store and talking to one of our salespeople in the lingerie dept for the best expert advise on bra fit.