Geometric reasoning and triangular coordinates

Question

The following is from a book:

I do not understand the sentence "... the point $(t_1, t_2, t_3)$ can be plotted by plotting $(t_1 = t_3, t_2 = t_3)$...", what is meant by the point $(t_1 = t_3, t_2 = t_3)$? How it is written it is $(t_3, t_3)$, but I do not think that every point $(t_1, t_2, t_3)$ could not be indentified with $(t_3, t_3)$?

EDIT: On next page in the book, see comments on Steven Taschuk's answer:

Answer 1

It says "$(t_1-t_3,t_2-t_3)$". (They're setting $a=-t_3$ in the previous sentence.)