How to divide a long tabular into 3 columns per page?

Question

I'm making a set of very long (1000 rows each) data tabulars. I'm using the longtabu environment, since it can split over pages.

My problem is that each of the tables is around 16 pages long, and I'll make three or more.

I'd like to reduce the tabular (I'm using \tiny for this, although maybe that isn't enough) and split it into three columns per page. By this I mean once the tabular reaches the bottom of the page, it continues on the next column of the same page. My problem is that the multicols environment divides the page in three, but longtabu only fills the leftmost column of each page, making it kind of useless.

I have also tried making minipages with a width of 0.3/textwidth, but minipages can't split over a page like tabu, and I would have to make every one manually.

This is my code so far, I've omitted the titlepage and most of the rows for briefness:

\documentclass[a4paper,reqno]{article}
\usepackage[spanish]{babel}
\usepackage{graphicx}
\usepackage[utf8]{inputenc}
\usepackage[margin=2cm]{geometry}
\usepackage{float}
\usepackage{tabu}
\usepackage{longtable}
\usepackage{multicol}
\graphicspath{{path}}
\begin{document}

\tiny

\begin{multicols}{2}

\begin{longtable}{|c|c|c|c|c|c||c|}

\hline

$A_{crit} / A $ & $P/P_{0}$ & $\rho / \rho_0$ & $T/T_{0}$ & $a/a_{0}$ & $M_{crit}$ & M \\ \hline

\endlasthead

0.000 &  1.000 &  1.000 &  1.000 &  1.000 &  0.000 &  0.000 \\ \hline 
0.009 &  1.000 &  1.000 &  1.000 &  1.000 &  0.005 &  0.005 \\ \hline 
0.017 &  1.000 &  1.000 &  1.000 &  1.000 &  0.011 &  0.010 \\ \hline 
0.026 &  1.000 &  1.000 &  1.000 &  1.000 &  0.016 &  0.015 \\ \hline 
0.035 &  1.000 &  1.000 &  1.000 &  1.000 &  0.022 &  0.020 \\ \hline 
0.043 &  1.000 &  1.000 &  1.000 &  1.000 &  0.027 &  0.025 \\ \hline 
0.052 &  0.999 &  1.000 &  1.000 &  1.000 &  0.033 &  0.030 \\ \hline 
0.060 &  0.999 &  0.999 &  1.000 &  1.000 &  0.038 &  0.035 \\ \hline 
0.069 &  0.999 &  0.999 &  1.000 &  1.000 &  0.044 &  0.040 \\ \hline 
0.078 &  0.999 &  0.999 &  1.000 &  1.000 &  0.049 &  0.045 \\ \hline 
0.086 &  0.998 &  0.999 &  0.999 &  1.000 &  0.055 &  0.050 \\ \hline 
%etc...

\end{longtabu}

\end{multicols}

\end{document}<code>

Any similar solution for this document would also be greatly appreciated

UPDATE

I have found a way around that works for me at least. It is a bit ugly, since it requires the tables to be compiled first.

First, I have separated the tables on their own document, half the size of an A4 \usepackage[margin=0cm,paperwidth=105mm,paperheight=297mm]{geometry} and compiled them in a single column.

Then in the main document I import them with pdfpages like this \includepdf[pages=-,nup=2x1]{Toberas.pdf}. Notice the 2x1 that will insert two pages side by side, virtually making two columns.

I guess if you make the tables smaller in a third of an A4, and select 3x1, you will have three columns.

I have uploaded the whole thing to my dropbox if you want to check it out, or if you want tabular data for nozzles and shock waves :). https://www.dropbox.com/sh/ib4ixilupq1vnb8/AAAxDsO63p8umbPKemxNYEEta?dl=0