This is a fairly straight forward suggestion. My main pets are 50, 49, and 49, and I was trying to grind them. I knew I was at Lord Talon in the Nightfall Bandit Dungeon, so I figured that he'd give good exp. So I bought a bunch of elixirs and stuff and beat him, but when I checked to see the amount of exp my pets gain, they got a grand total of 3.

With a battle that difficult, I really think all pets should get exp. So my thought is that opponents with 3+ stars give exp no matter the level of the pet battling it, because wasting a ton of healing items just to get three exp per pet is quite annoying.

This isn't just a part of Sylestia; it's common in many games. When your level is that high above an enemy's, it should be very easy to beat that enemy, and so the reward is much less. I've seen games where you get absolutely nothing for beating low-level enemies.

Honestly, lv50-ish pets shouldn't be having that hard a time against lv30 Talon... I've taken a brief look at your party setup, and I think that's part of the problem. Their equipment is low-level and mismatched (your Air pet, which is a strength element, has three intelligence-based items, for example).

You also don't have a light pet. It's not a requirement, but I know that most of the "veteran" players would recommend a light pet. It's a built-in healer, and while they don't deal much damage, any little bit of healing counts and reduces the amount of elixirs needed. I'd recommend switching either your air or fire pet to light (I personally prefer using air, but other people prefer fire. It's just a matter of personal preference.)

May I direct your attention to this guide on choosing elements and equipment?

All that being said, I wouldn't support getting experience from all enemies regardless of level. It would just make it too easy to level up fast and then smash through the story without enjoying it at all. I could maybe see upping the exp gain on elites and nameds, like Talon; but otherwise, it just doesn't seem necessary. Having a well-balanced party will solve a lot of the problems you described.

Frankly, if your pets are so much higher levelled than the opponent, you shouldn't need to be using healing items. That's why you don't get exp for it- taking out enemies some 20-30 levels lower than your team should not be difficult.

I went and had a look, and if your team is Emerald, Ruby, and Amethyst, then your problem is really your Equipment and prof setups (immediate example: two Physical element pets with Magical relics and a Magical element pet with a Physical relic is just not a very good plan for a start. It's severely hurting your pet's damage output potential)

(Water should be Tank, but you'd need to ask someone else about exact prof setup- I think Water tanks go with either HP or Str?
Water tanks should have Assassin gear)
(Air attackers should have Myrmidon gear and profs in Str and then Dex)
(fire attackers want Evoker gear and Int prof)

Okay, so I tried out what you two suggested and holy cow, it works out amazingly. Ruby is now light and I redistributed gear and I'm absolutely crushing these dungeons now.

I do still think dungeon bosses should give more exp just because they're, you know, dungeon bosses, but what you said my problem was really was my problem. Heck, after changing my set up I managed to take down King Decius (or however you spell it- Forgotten Tomb boss) with a few revival draughts- before I couldn't get him down to %50. So thank you so much.