If I turn Ninja's Kunai into a creature, can I then equip it with another Ninja's Kunai? And would it allow me to sacrifice the creature,for the ability, 1, tap, Sacrifice Ninja's Kunai: Ninja's Kunai deals 3 damage to any target?
If I turn Ninja's Kunai into a creature, can I then equip it with another Ninja's Kunai?
Yes, you can. In the definition of the equip ability, there is no limitation on what kinds of creatures an equipment can be attached to:
702.6a Equip is an activated ability of Equipment cards. "Equip [cost]" means "[Cost]: Attach this permanent to target creature you control. Activate only as a sorcery."
Any permanent that is a creature (like your "animated" Ninja's Kunai) can be the target of an equip ability.
There are such things as equip abilities that can only target certain kinds of creatures, but those will say so on the card.
702.6c Equip abilities may further restrict what creatures may be chosen as legal targets. Such restrictions usually appear in the form "Equip [quality]" or "Equip [quality] creature." These equip abilities may legally target only a creature that’s controlled by the player activating the ability and that has the chosen quality. Additional restrictions for an equip ability don’t restrict what the Equipment may be attached to.
For the other part of your question:
And would it allow me to sacrifice the creature,for the ability, 1, tap, Sacrifice Ninja's Kunai: Ninja's Kunai deals 3 damage to any target?
No, it would not let you sacrifice the creature. You have to sacrifice the equipment that is attached to the creature. That's because the card name "Ninja's Kunai", when used in the ability of Ninja's Kunai itself, actually means "this permanent", not just any permanent that happens to be named Ninja's Kunai. It's as if the Kunai's ability was worded "Equipped creature has '1, T, Sacrifice [this permanent]: [this permanent] deals 3 damage to any target.'"
201.4. Text that refers to the object it's on by name means just that particular object and not any other objects with that name, regardless of any name changes caused by game effects.