@Phantom Rider  

 
My asumption is that (since only 1 body is living at a time, at a rate of 1 second per second), their total lifeforce is simply the sum of their remaining life years/heartbeats [depending on the “fate” mechanics of their universe].  
. Yona is a young adult, so her initial fusion with Applebloom is a simple median of their respective ages (for simplicity, lets say magical little ponies & yaks live the same ammount of years), so they would have live for twice ammount of years, aging at half of the normal speed.  
. The next fusion with Scootaloo is NOT a median of 3 equals, but a combination of a “normal” pony with a “double” ponyak.
 
. Given the “special rule” mentioned by Applebloom, it seems the potion “counts” the total ammount of years ALREADY LIVED by both Applebloom & Yona as a simple sum, before applying it also to Scootaloo´s chronological age (that is the same as her biological one), then dividing it by 2 like before [so it is like if Scootaloo was fusing with a normal pony that was twice as older].  
-> But then the “biological” (aparent) age of the 3_fusion ponyponyak is still determined by piling up the “remaining lifespan” of the 3 entities and dividing it for the ammount of “souls” in the body (also 3).
 
Given this scheme (if only other fillies of the age of the CMC are added), they will never “reach” the biological age of Yona, but will get closer to it on increasingly smaller steps.  
Note: Fusing with an alicorn posibly stops the “aging” process completely (growing/developing is a different biological aspect).

Doesn’t that mean this is taking years off their life? Or will their combined power be enough to use magic to keep themselves young for a few zillion years?