Are you sure 3 subjects is worth opening a session for?
Regarding instrumentation, yes that is absolutely enough time - more than enough in fact. Radio navigation is also not a difficult subject either. In fact I think instrumentation and radio nav are similar in that they are just 'click and move' type questions, some very basic and easy principles need to be grasped and that's it, if you have difficulties remembering somethings then you can use the last resort of memorising a few questions by force ... nothing surprising in the exams for either of them either. They're different to say Gnav or performance where you have to actually understand what's going on in order to make the successful calculations and memorising questions won't help you.