My DD went to all three Seminars AND Girl State (plus a week long running camp). She was tired and home sick but learned a lot and is very proud of this accomplishment. My advice is to do all you can because this opportunity only comes once in a life time!!
Our DS made the same decision around the summer seminars, which we supported. And followed up with overnights to two of them.
It was worth the hassle & cost, it made his decision clear.
Last summer there was one combination of sessions that allowed for a reasonable schedule to hit all three. And another that would have worked, but required flying from one to the next. USCGA AIM is later, it's easier to work in if desired.
Even though it's not a total cadet experience, you do get exposed to cadet leadership through cadre. And a good feel for the school facilities. And some exposure to cadet life.
What impacted our cadet is he saw a significant difference in professionalism across the cadre. Not just individuals, at all levels. His initial observation was confirmed in his overnights.
That alone justified the expense.
My read: If you can't make the summer seminar for your choices, then for sure arrange detailed visits, and ideally an overnight if they allow it.
They will be committing the next 9+ years of their life! Make an informed decision.