How often should I suggest Mom shower? I’ve been doing every three days. Is that enough?

If you think so, and she thinks so, and assuming basic hygiene is maintained with washes in between, then yes it is.

I've really got nothing to add other than to point out the daily bath/shower is a fairly modern concept and not really necessary for health, just societal expectations. Where I live in AL they pay extra for more than one assisted shower a week, and in the nursing home they get two.