Are the Three Minds a form of idealism–the suggestion that reality is fundamentally mental? Perhaps surprisingly, the answer is no. But how is that possible? The Three Minds are minds, right?
In the Three Minds framework, mind, information, and energy are the flux of undifferentiated consciousness. Consciousness interpreted as bits conferring some degree of certainty is called information. Consciousness as an experience of being some way is mind. And consciousness recognized in its capacity for movement and direction is called energy. But consciousness-as-is is necessarily none of these.
This consciousness-as-is is what we refer to as "ultimate reality", which corresponds to the Third Mind–no mind. Here, even the English word "consciousness" falls short, but I use it because I believe it's the best word we have. To not use any word can be helpful in some situations, but given where society is today and how much work remains to be done to bring a better understanding of even grosser notions of consciousness to fields like healthcare, the word consciousness is needed.
Therefore, in the Three Minds framework, consciousness is reality. Its illumined, self-knowing, self-effulgent, self-experiencing, non-dual nature is the Second Mind. Here, as the Second Mind, the world of idealism begins. Loosely, this Second Mind may also be called consciousness because, again, the power of the word consciousness is in its function as an indicator–a trail of breadcrumbs from the First to the Second to the Third Mind.
When consciousness presents itself as what it's like to be, we call it mind. When mind divides itself and looks at itself, we call the aspect of it that is externalized in the process "matter". In this sense, mind and matter are not really dual aspects of consciousness. It is not that consciousness directly splits into two equally "real" parts: mind and matter. Rather, it apparently presents itself through knowing as mind, then subsequently forgets a part of itself as matter. Thus there is no independent matter nor mind. There is only consciousness.
The nomenclature of the Three Minds has been adopted specifically for the First Mind. It is, after all, the First Mind which seeks to know. And what the First Mind knows is the apparent separation and distinction of mind and matter. Accordingly, the First Mind climbs the ladder of the Three Minds, moving from mind-matter dichotomy to mind-unity to the Third Mind, at which point the Three Minds nomenclature falls away.
The Three Minds framework is closest to Advaita, but differs in some significant ways, as elaborated here.