It's possible even if it was those 3 times, that she felt dirty, cheap, as you say, she views your marriage , relationship of 30 years to be soulful, soulmates, you talk about everything.
Sub-consciously she may be holding on to a "feeling" that she can't shake because 30 years is a long time, or 25 to be with someone, and if you care and love that person, you sometimes want to tell what happened to be told, it wasn't your fault and therefore, to let it go.
It seems you feel that.
Let her know she can tell you anything, nothing matters as you two are soulmates and past is past... If she has something she wants to get off her chest, eventually she will, fear is stopping her from telling you...
If she said 90, at the beginning it could also be that something bad happened to her, with a so called boyfriend she trusted.