Three days would be tight to try and see much of more than one island, I'd have thought. Part of the problem of course is that you have to get to wherever the ferry leaves from in the first place, and without your own transport that'll be time consuming in itself. From Glasgow it's about a two & a half hour drive to Oban for the Mull ferry, for example, but once you factor in ferry timetables you really have to spend the bulk of one day travelling.

I've never been to Islay, but I've heard good reports, and there's no shortage of distilleries there. Reasonably helpful summary and links on the Calmac website - they also mention a whisky hopscotch ticket, if you can find the time!