It is claimed that the priest architects of Egypt could lay out a right angle with the aid of a rope divided in to a ratio by three knots.?

 3:4:5 it is ratio

That is not a question, unless you mean it as a true/false statement. It has been suggested (not proved to my knowledge) that such a method was used in ancient Egypt. The procedure may have been in use long before proof of the Pythagorean theorem. It would require the rope to be divided into ratios of a Pythagorean triplet, probably 3-4-5.

As for three knots, I often see it stated that way, but why three? A rope could be divided into three segments by two knots. Fraying at the ends would make four knots work better.

Find a rope. Tie a knot in it 3 units from one end. Tie a second knot 4 units from the first knot. Tie a third knot 5 units from the second knot. Now, you have all you need to lay out a 3-4-5 right triangle.