Open Collective (OC) is great! It's primarily a payments platform.

Open Source Collective (OSC, which is related to OC in convoluted ways I don't fully understand) is a fiscal sponsor of OSS projects, and is also great. :^)

Open Source Endowment (OSE), on the other hand, is a pile of money that earns interest that then gets distributed to OSS projects. So conceptually some projects either fiscally hosted by OSC or using OC as their payments platform could receive funds from OSE.

Does that help?

Edit to disclaim: I'm on the OSE board.