Clearing Chapter 3 of the Campaign on Normal is not very challengin if you follow several important steps. 

The element of the enemies in the chapter is Earth, so take 1-2 of your Fire invokers that you have in collection and even if they are common or uncommon level them up to level 30+. 

Do not forget to equip relics on every Invoker in your team in the Invokers menu.

If you have 3 or 5 reclis from 1 set (Attack or HP) you will get a 15%-45% bonus to attack or HP so make sure every invoker on your team is equip with relics. 

Try to equip ATK% and HP% relics on your team, focusing on Attack for Attackers and Attack + HP for Tanks and Healers. 

You can use Auto-Equip at this stage. 

Here's a playlist with video guides where to find all chests on all levels to get 2nd star in each level. 

Invokers: Titan Legacy - Campaign - Normal - Chapter 3 - All Chest Locations