The Curious Case of Disappearing Funds
I am not detailedly aware of European Training Networks, but it sounds like you are grossly misunderstanding some things here.
1. €200k for three researchers for three years are absolutely certainly not only for your travel and consumables.
As you correctly assessed, this would be very lavish and no EU project that I have ever seen gives you that level of funding. European Training Networks in particular are, as far as I have seen, generally underfunded when it comes to travel, and assume that the host institutions also chip in. Assume that there are costs calculated in here that you are not seeing, for instance institutional overheads or the costs for organizing trainings.
2. I understand that the "lack of transparency" is disappointing, but this is not your project, it's your supervisor's.
Quite frankly, PhD students generally have no detailed insights into the finances of the projects that they work on because they are also not formally responsible for them (i.e., you are probably not formally responsible for signing expenses).
Your supervisors report to their administration and to the European Union, not to you. They do not owe you an Excel file of where the project finances go. But do not worry - there is a formal audit at the end of the project which will look very carefully into where the different funds went. If it was actually grossly misspent (as opposed to spent on things that you or the other PhD students didn't want), legal issues will come up. However, I presume this will not be the case, as your supervisors are almost certainly aware of the mandatory end-of-project audit.
3. Project politics are a thing.
Your university presumably commited to sending you to certain trainings when they signed up for the network. Yes, it sucks if they are unrelated to your work, but sometimes you'll have to do things for your project that are not key to your personal research agenda. In a Training Network those are a few trainings here and there - in an IP project it could have easily been as much as 50% of your work time. Without knowing the details, I would still guess that you are complaining on a fairly high level here.
That said, your institution still screwed up.
There should be money for you to go to conferences and present your research. If the project funds do not suffice for that, they should have made sure that the university can co-finance from other sources. If they are unable to do that, they should not have hired a PhD student.
These are public funding from the EU, I don't think there should be any secrets how the money has been used, because eventually the universities involved will have to report this to the EU. Don't go point blank to the secretary or the PI until you have gathered more information, there might be expenses or costs that are legitimate and that you don't know about.
A first step would be to ask the secretary/auditor in charge of the bookkeeping the documents documenting how the money has been used.
Secondly you have to go through these and flag transactions that according to your knowledge don't sounds right. You then need to gather some information if these flagged expenses sound right or not, you could also contact the EU for this.
Third step, if some of these transaction don't make sense ask to the people who reported these expenses.
Last step, if you find discrepancies you have to bring this up with the EU.