Triple bar- get to the base. Ideally the last few strides you'll open them up a bit so you can gallop right up to the base. It's the same idea for a open water, with a bit more gallop. With an oxer you'll want more impulsion with them more on their hocks . If it's ramped you can get a little deeper than if it's square because they have a little bit more room to get their front legs out of the way . If your horse is used to being "dropped" than typically when their front feet hit the ground you can drop them so this creates a pause where their hind legs are able to create more power upon take off. But know your horse and if this suits it.
Try to ride the same way you would when heading to a vertical, but add a little more leg up to the base of the jump. Perhaps slightly widening your hands to prevent running out and to keep your horse straighter. Pretend that it's the same as any other jump - if you get nervous or ride differently your horse will pick this up. Try to be as confident as possible and actually ride and give direction rather than just being a passenger :)
It depends on the horse, but getting out in front of your leg is key. Then you can adjust lightly with your hands and legs if necessary to get to the base. For oxers and triple bars, getting right to the base is the way to go so that they can clear the spread. When going to a water jump, I get into a driving seat with my horse in front of my leg with a good feel on the mouth to fully encourage departure from the ground.