you need to feed the hero you want to avacne euql to the current number of stars sacrifical herooes with the same amount ofs stars

A 3 star herp needs to be elvel 30 and fed 3 level 1 3 star heroes. A level 4 needs to be 40 and fed 4 level 1 four stars and so on.

That's why 6 starring a hero is such a significant investment.

Go to the tavern, select your level 30 champ, there will be 6 slots, 3 open, now you need three 'food' - three 3 star guys you don't want or need, selecting these will place them in the open slots, when you have click the 'upgrade' button, your guy will go from level 30, 3 star to level one four star

If you haven't got three then use green shards to summon, you'll get one, two sometimes 3 star champs, if you have level 2 select them, select upgrade level then select the one stars, when the 2 star is level 20 select upgrade rank, pick two two star guys and level it up to a 3 star

Thanks for answer, need more 3 stars heroes

Keep at it! You'll get there. :D