I hope you're 8 years old or something because that was cringeworthy to read.
Nope, i am 17. And an 8 year old would write broken English like Temmie. Teenagers played Terraria like me know any strategies. I haven't used one in any Expert Mode world. Call me a boss, but i played terraria for about hundreds and thousands of hours.
You know, that actually hurt a little bit of my taste. But it's okay. I can make better advice for you.
It's recommend if you mine from Cobalt/Palladium to Adamanite(?)/Titanium, then you set up an arena for these 3 mechanical bosses (I recommend Fighting Destroyer first because that i usually do. Plus, this is the easiest boss for me because i dodge for legit.), and make sure you have enough Greater Healing Potions, Pressure Plated Heart Statues, and Heart lamps that you may really need for the fight.
Secondly, i recommend you to fight with in platforms because it's very useful for beating Destroyer, The Twins, and Skeletron Prime. If you are really careful, you can beat it all in 3 nights. Me? Only three nights because i do it one at a time in Expert Mode, sometimes in 5 nights. Plus, during Expert Mode, you might find the enemies hard as hell to defeat, so make sure you got the best armor you have and the best weapons you have. Otherwise, you are better off in a pile of bloody entrails everywhere.
Plus, my English is perfect, although a few mistakes in spelling and grammar that i fix. Except the main ore from 1.1, boy that was hard to spell it!
P.S: If you are melee, it will be harder for you to survive in expert mode in Hardmode. So, keep that in mind. Plus, i know that it's very cringey for you to read this, is because i didn't have much time while playing Terraria by a lot. That's the reason my writing can be cringeworthy for some people.
Now i gave you some advice, you have better chances to survive in Hardmode or what i call Xenomode (Because fan of Xenoverse??? No idea.) and an even better chance of defeating 3 bosses at night in random nights with a setup arena. Now you know how to do this, you can survive like a Master Terrarian above the level of Yrimir.