Do i need to buy (3) of these trackers? Two for ankles and one for hip?

If you're talking about Full Body Tracking for per say VRChat, then yes you need 2 for each foot and one for the hip.

Do you need a computer for this lol?

Yes. It requires Steam VR to use

No, the new Tracker is compatible with both the original Vive and the Vive Pro. In addition, its currently compatible with Lighthouse 1.0 basestations, and will be compatible with the Lighthouse 2.0 standard.

the new htc tracker and used htc tracker all have all the accessories? Like Power cord, and usb plug.

Sometimes it doesn’t come with everything, one of my used trackers only came with the tracker, which doesn’t make sense because it’s completely useless without it and there’s no replacements for the accessories. So if you end up only getting the tracker, make sure you return it ASAP