2500 grafts FUE; Can double hair grafts (or triples) be placed with singles to make the hairline look more natural and dense?

Generally, single hairs are used for the hairline while doubles or triples are used behind the hairline. If you are dissatisfied with your results, a second surgery may be an option to correct the hairline.Keep in mind that it may take 6 to 12 months to see the results of a hair transplant surgery. The results depend on a variety of other factors such as how long you keep your hair, how you style your hair, the hair color to skin color contrast, curl of your hair, the thickness of your hair.  If you have very fine straight hair that you keep very short, a hair transplant surgery will NOT look good.
The overall success of a hair transplant surgery depends on: (1) artistic and technical skill of your doctor and team (2) communication between you and your doctor to establish a realistic goal.

Only single grafts should be used for the hairline, doubles and triples mixed in will look unnatural 

In the hairline, the best choice would be to use single hair grafts. Combining two hair grafts and single hair grafts  to create three hair grafts in the hairline would very likely look unnatural and would not be recommended. However, if some of the more posteriorly located grafts (i.e. a few cm behind the hairline) were combined to make three hair grafts, this could look acceptable as it would not be so noticeable in this location and potentially give more density.
If this does end up looking unnatural, some of the grafts could be thinned out with FUE or some single hair grafts could be transplanted around the thicker units to camouflage them. 

From my experience, single hairs are the gold standard for the hair line. If you have very thin hair, doubles may be acceptable. If you have a hairline of transplanted thick double hairs that ends up looking “transplanted”, you can have a small micro session in the future to plant single hairs below the doubles to improve the hairline. Best of luck. 

If you had two and three hair grafts in the hairline, the hairline will look transplanted because that is not a natural happening.