Yes, it's totally allowed to use the free camera to fly around the map. There are no limitations set for the console (or free camera) as most of the commands don't work in multiplayer. For instance, when playing in multiplayer it's not possible to spawn your truck by pressing F9. So as the commands and functions that could possible cause trouble in multiplayer have been removed from the console, you can use the free camera with no hesitation.
And as mentioned in a message above, you can use the free camera to check TruckersMP ID if you notice someone breaking the rules and you didn't manage to catch his/her ID, for example. However, bear in mind that you cannot just fly around the map with the free camera to report everyone who breaks the rules (like, for example, a Game Moderator could possibly do) as that's against the rules.
By the look of things, it seems like that the road you linked above is part of the base game and thus, the Vive La France DLC isn't required for the training road you've planned. However, you can always double check that by disabling the Vive La France DLC in your Steam library and then go in game to see if you're still able to drive on the shown road.
The easiest way to get a triple trailer in Euro Truck Simulator 2 (works in TruckersMP as well) is to use the Forerunner's Additional Customisation modification. You can get it either from the Steam Workshop or from Forerunner's own website that you can access in here. There are also other ways to get a triple trailer, but using the Forerunner's modification is clearly the easiest way. If you're not really satisfied with the selection of different triple trailers that you can create with the Forerunner's Additional Customisation mod, you can always search for different triple combos on the community mod sites (e.g. https://ets2.lt/en/?s=multiplayer), but remember to double check the §3.2 - Trailer modding rules before using them as there's always a little chance that some of the comunity-made triple trailers aren't allowed to use in the multiplayer...
When using the mods, you will need to first open up the game in single player, go to the mod manager and activate the triple trailer mod that you want to use (you should be able to get the trailer from the trailer dealer - at least when using the Forerunner's mod - while playing in the single player). Then open up the multiplayer (TruckersMP) and you should have the triple trailer.
You can also create a triple trailer with using save-editing, but if you're not really familiar with it, I'd highly recommend using the available triple trailer mods instead. Anyway, if you can't find any exsisting triple trailer modification, you can always check this topic to get some more (more or less advanced) information about save-editing:
Hopefully this answered your question. If you need further assisntance, feel free to ask. Other than that, have a great day!