Swiftees already know Taylor filed docs to trademark her first 2 cats -- Meredith Grey and Olivia Benson -- but she didn't want to leave the new cat out in the cold. That's right, she's now added BB to the squad.

According to the new docs ... Taylor's applied for the trademark, "Meredith, Olivia & Benjamin Swift." The legal move now opens the door for Taylor to cash in on every kind of merch imaginable -- clothing and phone accessories, toys, luggage, home furnishings, key rings, arts and crafts kits, ornaments, glassware, coloring books, magazines, etc ...

You get the picture ... she can now milk these cats for all they're worth, and then some.

And yes, cats have nipples.

Meredith and Olivia made their merch debuts last year ... so this is old hat for those cats.

Taylor adopted Benjamin earlier this year after they met on the set of her "ME!" music video.