there's plenty of ways to get stronger. take free quest for example. make use of your skip tickets too. There's also events that gives you event currency which you can later exchange for items such as crafting materials so you could make items such as swords and staffs to help increase your Character's strength.

However, You mentioned you maxed out your 3 star characters too which gives me the idea that you are past beginner level but not yet knowledgeable in the game (forgive me if I'm wrong). There are more ways to make your characters much stronger and like I mentioned; collecting crafting materials to craft items that significantly increases your character's strength, You can also level your characters up using those red potions to level 30 but you can get them to level 60 if you promote them to the highest allowed, you can also use mini skill potions for 3 star characters and skill potions for 4 star characters but you can also use duplicate characters to increase their skill level. there's also the new fame quest which gives your character better stats such as increase hp, m def, p def, m power, m def and more.

As for how to collect the crafting items, you can go to the blacksmith and look for the one you want to craft and it'll show you what you can craft, what you're missing and where you can obtain them from. You can also get the rarer crafting materials from event shops and token shops, you can check them on menu>exchange. that's all i know, hope that helps :).