What I would do is create a separate history table. Each would have, say, its own primary key, the key assigned to the machine, the date it was activated, and the frequency. You could throw in any other information related to each specific activation, as well - who, what, where, when, why, whatever.
Normally, you would just leave that information alone, and use a query to just pull up the three most recent occurrences. Doing it that way lets you look back through the entire history if you need to, while not overloading the user. (Personally, I'd just use a form/subform setup, so that someone could look at each machine and see its entire history.)
If you really want to limit the history to the most recent three activations, then you can create a delete query that will delete all but the three most recent activations, but that's going to take a couple steps. Basically, you make one query that pulls the top three results from the history table that match the specific machine, sorted by descending date. Then you make a delete query with a left join on the query you just made (ie - properties will be 'all records from history and only matching records from' the query), have it select all the records from the history that match the machine's ID but whose history ID values are NOT in that other query.
Clear as mud?
Edit: I've attached an example of what I meant by a form/subform setup to show the history. Sorry, can't access Imgur from work, or I'd host it and just embed it in the post.
