"...using -Destiny Hero- monsters you control as Fusion Materials..." One D-Cubed is treated as D-Hero but the other one not. You should use ONLY D-Hero monster and cannot use other monster thats not D-Hero
I just crushed a ssa user with this, really good card if you use it properly
1: Vision HERO Trinity is not a D-HERO, so it can't be fusion summoned by this card.
2: D-Cubed only counts as a 'Destiny HERO' monster when normal summoned. Trinity needs 3 HERO monsters. Only one of your D-Cubed counted as a HERO monster at that time. (This is based on the assumption that you used D-Cubed's effect to special summon the other copies.)
This card can Fusion Summon non-Destiny HERO Fusions, it just requires all the monsters you use to be Destiny HEROes. So if you controlled 3 Drilldark, for example, you could use D-Fusion to Summon Trinity just fine. (Heck, Summon a D Cubed, equip another D-Hero with Dragonic Attack and you can Summon Panzer Dragon with this thing.)