RD, as Jim says, it's easy to merge the 3 files, so here's what that looks like (1st photo). As you can see, they don't line up too well, so you have some work to do.
- The Black tracks are from your Plan#66.rrt file and are on layer "track 1".
- The Blue tracks are from your Plan#66-2.rrt file and are on layer "track 2".
- The Green tracks are from your Plan#66-3/rrt file and are on layer "track 3".
FWIW, rather than change the individual track colors, I generally change the color of the layers instead. In this case, I would also add a layer for the grades between the levels and set that layer to a 4th color.
Based on the drawing you posted from the magazine, it appears that the Blue tracks should connect to the 4 tracks in the larger blue circle and the Green tracks should connect with the 2 tracks in the smaller blue circle. I couldn't tell from the drawing exactly which layers were which, but it looked like the Green tracks were on the lowest level and the Blue tracks on an intermediate level, so I set the height of the Green tracks to 0", the Blue tracks to 7" and the Black tracks to 14" just to show some separation in the 3D view (2nd photo).
The 3rd photo shows the beginning steps to link Level 2 with Level 1 (Level 3 is hidden). As you can see, I added some straight sections to expand the Blue curves on the left so the loops on the right sit more directly below those above. More work needs to be done so they all connect and I thought I'd let you play with that. Once that's done, the expanded curves will then form the grade down to this level. I did create a grade with the one leg I connected. I added a "grades" layer and changed the color to Cyan (looks like purple in the photo). The grade goes from 14" down to 7" and by including the straight sections, the slope is an acceptable 2.9%.
I hope some of this helps you move along with this design. I assume you have the 14'x25' space the design will need for O scale.
NOTE: I didn't mention the difficulties you are going to have reaching various sections of the track, etc., so you really need to think about access if you pursue this.
