You need 4 separate tie downs, one for each corner of the machine, each capable of holding the machines weight. This way if one falls off or fails you still have it secured. You also need a strap over the loader bucket securing it to the trailers deck and one over any implement on the 3 point.
So to answer your question, you will need 6.
Here is what I do with my tractor which is about 5200#. On the rear I have two pieces of G70 5/16" about 8' long chain pulled tight by the tractor once properly adjusted. On the front I have two more of the same chains but I use lever style load binders to snug it up.
I then use two 2" wide 10,000# straps with a Work Load of 3,333# to go over my loader bucket and implement in the rear. Just be sure to use a folded towel or old carpeting to protect the straps from abrasions.
Chris