You can adjust the loft on the 15* and 18* and hit them on the range. See what happens! And try choking up on the shaft if you feel it is too long. You can always shorten the shaft though you will change the swingweight. Try putting the 21* shaft in the 18* club and the 18* degree shaft in the 15* club.

You are probably the best judge of which of these you use the least and which one has a manageable gap that adjusting one degree will make a difference. You can always change back and forth depending on courses played and/or course conditions.

The SureFit hosel for all metal woods will create a more closed face as the loft is increased. A 15 degree adjusted to 15.75/16.5 degrees will have a more closed offset and 18 moved to 17.25 will be a more open offset. Your stroke will determine if a hook or fade will be more problematic for you.

I'd drop the 18, and weaken the 15 to 16.5 degrees (not sure what that setting is off the top of my head).That way your gapping would be better. If you got rid of the 15 you'd have a 7 degree gap between driver and the 18, assuming you cranked it down to 17.

7 degrees is a large gap between clubs.

Given you have woods of 15, 18 and 21 degrees you have many options. You can weaken the 15 and strengthen the 21 to blend gaping.

You can strengthen the 18 and weaken the 21 for gaping as well.

You could also weaken the 15 and weaken the 18 or strengthen the 21. The shaft length won’t make that much difference, it will a little but not as much as you think.

If you really wanted to go down the rabbit hole, you could put the 21 head in the 18 shaft…. Same with the 18 on the 15 shaft. The longer shaft will get some distance back and then if you play with the loft you can come up with another combination.

Possibilities are endless….

Spend some time on the range adjusting all of them and find the best combination that works for you.

Id weaken the 15 to 16.5 personally

Without knowing what the next club in your bag is (5-iron, 4-iron, something else) this really is something you'll have to figure out on the range. If we know what your next club is, then we could help by doing the math based solely on loft numbers. As has been mentioned above though, some range time to figure out what you want each club to do and how that is done best for you is what you most likely need to do. Being your 15/18/21 are all Titleist TSR4s then swapping shafts and adjusting lofts should be very simple for you to get the gap coverage you need. It will take some range time though - or find a simulator and use the data to help get you what you need.