Steve S -
Great questions - I had the same ones for a long time before I finally got it all sorted out. And terminology is important to get answers when you have a question. I'll try to help:
First, 3-point refers to the attachment/mounting points on the tractor. Sleeve hitch is a type of hitch on implements/attachments (most common, Brinly).
A Cat 0 is a 3-point hitch, most often found on larger, super garden tractors, but also available from Xtreme Motorworks (a sponsor above) and other sellers. Here's a picture of a Cat 0 from the Xtreme Motorworks website (thanks, Aaron - I hope you don't mind!).
Any implement with a Cat O type hitch will work with this.
An IH 3-Point hitch looks like this (the "3 points" are circled in blue:
Although some implements may mount directly to this 3 Point, the most commonly used, Brinly-type attachments use a Sleeve Hitch, so you need a sleeve hitch adapter to use them with the IH 3 Point.
Here's a picture of a sleeve hitch adapter mounted to an IH 3 point:
The biggest advantages of a Cat 0 are that they can apply downward pressure, and they look really cool!
The IH 3 Point with a sleeve hitch adapter is less expensive and sleeve-hitch implements are generally easier to find (and less expensive).
That's my $.02 worth - I hope it helps. I'm sure others will share their thoughts.