However, using pufferfish, I can only tween two enclosed curves. It doesn’t allow to branch two curves together with a third larger curve at the bottom.
You could do one tween for each of the top curves with the bottom one respectively, loft them, cap them and do a boolean union, but something a bit more interesting would be to look into minimal surfaces like this How to make minimal surface like Daniel Piker? - #7 by Nick_Bruscia , there are countless tutorials on the internet for this kind of thing with grasshopper, so learning to properly use search engines for this purpose will help a lot in the future