Sorry about your dead one. Some of the smaller ones you buy at Lowe's etc seem to die back early, but this is pretty early for that.

Is there good drainage in the container? Are the other two looking healthy? I don't really have any answer, sorry.

I bought a potted dahlia at lowes. It did great for about a month. We've not been having a very good summer. Not much warm weather. It gets watered regularly,as needed. After two sets of blooms,the leaves started turning yellow then dying. (It's not being over watered) The whole plant is mostly gone now but blooms are still trying to appear. They just never get there and there's hardly any leaves left now. It's not trying to get new leaves any more either. I don't the pot is too small. What's up with this?