The BelleStarmon is last digimon in Three Musketeer. In BT6 we also have black option card Gewalt Schwarmer, 7 cost to play. As long as we have a [Three Musketeer] type digimon in the battle, we can play it ignoring color requirement. This is a very strong option card for a Megazoo deck (even if you need to pay 7 cost to use it) as it destroys all Digimons LV4 and below, and some LV5s. It's a good way to deal with rookie rushes or wide-boards. Too bad though it cannot be activated from the security.
Meanwhile, regarding the "Seven Demon Lord" Digimons: Lucemon Fallen Mode, Leviamon, Demon, Belphemon Rage mode, Barbamon, Beelzemon and Lilithmon, we already have 2/7 of them released in prior sets: Beelzemon in BT2 and Lilithmon in BT3. Beelzemon is a popular card back in the BT2 meta, and Lilithmon is a recent hit with Lilith-loop engine (with Omegamon Zwart) in BT5 meta. Now with BT6, both Lilithmon and Beelzemon will have better support from new LV3 Impmon to retrieve [Three Musketeer] of [Seven Demon Lord] from trash. Not only is this one of the the easiest and cheapest ways to return a LV6 Digimon from your trash to hand, but also sets up a warp digivolution candidate for BT2 Beelzemon. This impmon will improve the consistency of corresponding purple decks.