Level 3 heroes max out and their level won reach the top, it's frustrating me

You have to keep feeding them - once at 3/50 each one star gives you 10% chance of raising level, so 10 one stars means 100% chance of level going up, each 2 star 20% etc

Thank you never knew that…

Just train 10 uncommon heroes of the same color of the hero you want to max out entirely, press level up and it should allow you to upgrade their special skill level by 100%. If you use common heroes or heroes of a lesser color, may take you a fewer goes as you won’t hit that 100% guarantee.

Cannot remember what is involved in levelling up special skills of 3* heroes, but feeding them same color increases your odds over non same color. I am sure you will figure it out.

Have a peruse of this thread: