We did all three on our trip to Ireland last summer (which was not much longer than your trip). All three of these were the #1 top highlight of our trip (which also included a lot of other amazingly great stuff, so these were all really great).
We decided that for this trip, we were going to concentrate on the west/southwest of Ireland, arguably where the best concentration of things that most interest us are located. We decided to entirely skip Northern Ireland, the NW, the center, the SE and even Dublin (except for transport connections) for this trip. Our logic was that those other areas deserve time too, but were not compatible with a trip that included all the "wild Atlantic way" places that were at or near our top priority. I will add that the pace of our trip was fairly aggressive but we got it all done, had time to savor each of the places we thought we would love (we did) and it worked out great for us. We will go back and get to other parts of Ireland another time.
So...you asked about our three favorite places, which two to cut? Ouch, like losing fingers or children. I'd have to say...cut Northern Ireland instead, save it for another trip (no disrespect intended to Northern Ireland).
Practical considerations: If you are planning to do Dingle (which you probably are, if you're considering Great Blasket Island), and you're even thinking about Skellig Michael, that means you are going to spend time in the far southwest. It takes a bit of time to get there (and back).
The Arran Islands take less time to get to/from (but deserve at least one overnight - we spent 2 nights on Inish Mor which was perfect). They're close to Galway (which isn't that far from Dublin) and quickly accessed (take the plane, not the boat, for a dozen good reasons, not the least of which is that's far faster than the ferry and gives you more time there).
Skellig Michael is the geographic outlier (in all ways), and in fact is quite "iffy" - there's no guarantee the boat will go, they make that decision each day...but is well worth it if you can pull it off.
Great Blasket Island is a full day side trip from Dingle (and Dingle is already geographically somewhat of an outlier, too, just because it takes a while to get there).
So, I do not envy your dilemma. I'd only say that all three were amongst our favorite, most memorable places we have ever visited (and we've been around, so I don't say that lightly). It's hard to say since you have not posted details about the rest of your trip, but if it were me, I'd cut other things, and try to get to all three of these. I know that's not what you asked, but having been there 6 months ago, that's how I'd go. Good luck.