I personally think that DH is weaker than betty crocker so that's what I've switched to. I personally wouldn't use it for a 3 tiered cake. You could try lowering the amount of water to 1 cup and adding an extra egg per mix. That may make it strong enough to use.
I only use DH and have made plenty of 3 tier cakes and they hold up great.
Getting to the rest of your question, this subject was brought up here a few times, but long ago. It's rare to find those cake mixes in the UK. I hope someone from that side of the pond can help you, but you might have to resort to baking from scratch, instead.
Is there a particular flavor you would like to bake? Check in the recipe section. Almost all the recipes there are tried and true.
Theresa 
I use DH all the time, no problems here. My customers RAVE about the look and taste of their cake. If it' ain't broke, don't fix it
Jennifer
I only make wedding cakes for family, and because my scratch recipes are too moist and don't cut sharply, I use Duncan Hines. I am very inexperienced with wedding cakes. I made a four tier cake in May with each tier about six inches tall. The cake held up perfectly and I LOVE Duncan Hines taste. I scratch bake because I love the science of it, but DH is in my pantry too.
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