Is there a mathematical description of three-part ratios? Rational numbers have many interpretations, but one of the simplest is as a ratio of one number to another. The fraction $1/2$ can be interpreted as the ratio 1:2 (i.e. one apple for every two oranges). Rational numbers are also considered an extension of the number system of integers $\mathbb{Z}$ to (nearly) close it under division. 
I would seem like a natural extension of the number system to include ratios comparing three or more quantities. Is there any mathematical description, in terms of a "number system" or otherwise, of ratios which consist of three or more parts, such as $1:2:5$ (i.e. one apple for every 2 oranges for every 5 papayas)?

To represent a multi-part "ratio" $a_1:\cdots:a_n$, where each $a_i$ is an integer, I would suggest an element of the projective space $\mathrm{P}_\mathbb{Q}(\mathbb{Q}^n)$ (see Wikipedia) which is the set of equivalence classes of
$$\mathbb{Q}^n\setminus\{(0,\ldots,0)\}$$
under the equivalence relation $\sim$, where
$$(a_1,\ldots,a_n)\sim(b_1,\ldots,b_n)\iff \text{there is some $\lambda\in\mathbb{Q}$ such that }a_i=\lambda b_i \text{ for all }i$$
Denoting the equivalence class of $(a_1,\ldots,a_n)$ as $(a_1:\cdots:a_n)$, you can rigorous statements like
$$(1:2:5)=(3:6:15)\qquad (1:1)=(7:7)=(\tfrac{1}{3}:\tfrac{1}{3})$$
However, this is not really a "number system" in the same way $\mathbb{Q}$ is (it has no natural ring structure).
