SeaSonic is an OEM, a brand, and a designer.
Corsair and Antec are brands and designers, but neither of them manufacture their power supplies. The various units in their line-up are manufactured (and sometimes designed) by other companies and branded for sale. SeaSonic is one of Corsair and Antec's OEM/design partners, so there is a good chance that a handful of units in the line-up of all three brands are identical!
At any rate, because of this, comparing the brands without specifying specific units or at least lines (ex. Corsair AX series vs. Antec HCP, fight!) isn't really a meaningful thing to do.