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In this case, you pretend that you actually received 3 separate K-1s --  one for ET, one for USAC, and one for SUN -- and enter accordingly.  The only other nuance is that you don't get separate Part I or II info for USAC and SUN, but its not really needed.  Enter ET using the Part I and II info, and then for SUN and USAC  use the FEINs provided and in the Partnership name use something like 'Energy Transfer, reporting USAC'

Note that the reason you have to do this is that losses from one MLP (e.g., SUN) can't be used to offset income from another MLP (e.g., USAC).  So even though you own all three through your ET investment, for tax reporting you have to keep the gains and losses separate.