Quote from void_nothing »Hey, we're in the same room, you could have just asked me.
If that's the case, the equipped Sunsinger and equipped Elemental will each be both named Cylian Sunsinger and Quicksilver Elemental. That means that both abilities resolving will apply to both of them. So the equipped creatures will get +6/+6 after both abilities resolve and the rest will still get +3/+3.
For both the Sunsinger and the Gooey Ability Gainer Thing, that creature has the same name as every other creature that has a name, excluding the names of legendary cards. If that ability resolves, once with the Sunsinger source and once with the Quicksilver source, both of which have all nonlegendary creature card names, then those resolutions will both do the same thing, and all creatures that have some nonlegendary creature card name will get +6/+6.