So what I gathered is that a 100/100/100 will be possible if I were to fuse my 90/90/87 with, say, a 90/90/84 pet? It would just take longer to get that last ability to 100 since feeding costs more for divine? I just want to make sure I’m not capping my pet level on accident for that third ability.
I previously messed up a legendary pet by fusing too early and it maxed out at level 84 for the first ability. I just don’t wanna screw something up.
Ah, no, my apologies. You made me hesitate. I checked the wiki, and the page describing all things pets has updated. It now more thoroughly describes the process, offering more details. No, you’re going to want to max them both to be a 100/100/100, it looks like. I’m maxing my pets anyway, so it wasn’t a worry for me until now, but yeah, feed ‘me ‘till they burst! XD
This game has come a long way since I first started… and I’m not even a true OG. I’m grateful for the evolution! 
@Isabell
For the best information, check the specific wiki pages on “pets” and “fusing.” There you will find everything!
To fuse to a 100/100/100 divine, you only need two pets with 90/##/##. As long as the first stat is maxed on both, you’re fine.
This is true but it will be very expensive famewise to max the second and third stat, especially if you dont max them before fusing. I made that mistake, and my abilities are in a bad order so I’m struggling to get enough fame to feed my pet. I highly recommend maxing at least your first pet’s heal and mheal, as 1) you will be using it for a while before you max your second legendary and 2) it will decrease the amount of fame it will take you to max the stat once you fuse
Fusing a 90/90/X with a 90/90/X is Fame efficient (this is the recommended choice). Fusing a 90/X/X with a 90/X/X is FP efficient.
To further elaborate on Gamma’s point:
Keep in mind that any feed power currently entering your pet’s third ability could have been going towards its first ability, had it been a Divine. Additionally, the only reason a Divine commonly requires more FP to max is due to the higher levels costing more: to get from lvl 87 to 90 on a Legendary costs an equal amount of FP as on a Divine, but there’s a notable fame cost increase for the latter.
All in all, you really needn’t max your third ability before fusing. As for whether to max your 2nd pet’s second ability, that may be worth it considering that it’ll otherwise get lowered to the average of (90 + 2nd pet’s 2nd ability’s level) / 2.
As a F2P and P2W, I would want FP efficient because the FP choices are very limited (despite of login calendar).
For me to feed a Divine (I said it and I will say it again), I feed any items that have at least 1,000 FPs such as Bloodshed Ring, Magical Lodestone, Ogmur…
Basically, 1:1 FP to fame cost ratio when hitting to Divine.
I highly encourage you to be fame efficient then. I made the mistake to fuse early and the cost to feed a divine is devastating. If you value your FP to fame ratio 1:1, it is actually more reasonable to fully or semi fully max your legendary pets with ~500fp items(which will solve your limited fp options problem), then fuse for divine.
Ah, so WC incs. No problem. But my belief of feeding threshold for Legendary is 700 (can be 650) to 999 FP.
Point is, unless you are straight up buying all of your fp items with gold, being fame efficient is also fp efficient.
Honestly, as an opinion from an anti-pay-to-win player, I’ve gone down the route of maxing out a pet before fusion. I’m currently partway done with my legendary, and have no regrets from waiting to fuse my uncommon and rare pets maxed. Fame it’s worth its weight in gold to me.
(I wish I had electric as an ability, Heal, Magic Heal, and Rising Fury will have to do until I find a better option. Still a half decent find for obtaining all my eggs randomly in the wild with an awesome rare canine find in possession that setup?)