If it was so easy everyone would be able to do it and what is the fun in that? It’s a challenge for a reason! Eventually if you continue to play you will be able to beat all 3.
My best advice is to take axes, bombs, dragon flag and mana. Or you can take arrows instead of dragon flag or mana.
Don’t use any of those items until the final stage. If possible try to have your hitters, gormek and hu Tao supers ready to go too!
Once it starts use a dragon banner, then axes, bombs, special attacks then dragon banner again if it’s been 5 turns. Then mana regen gormek and i would attack Guinivere first. Rinse and repeat with mana regen and gormek and attack the other 2! While using boards as well!
If you use arrows instead of mana or dragon it’s really pick your poison, dragon flag is good for board and special attacks, mana gets you to ignite the special attack.
For me i scored best using mana, dragon attack, axe and bombs.
My heroes were Wu Kong at 7 hero class, Summitmmo fully leveled, Boldtusk at 12 hero class, Caedmon at 8 HC and Tiburtis fully leveled.
Good luck and i hope that helps!