Having trouble to defeat bosses? Here are some tips!
Eye of Cthulu
Choose a flat terrain, place a campfire, and have at least a Cactus/Shadewood/Ebonwood armor, some potions and some Shurikens/Throwin' Knifes and a Spear ( not necessary). In his first stage, just throw the Shurikens/Throwing Knifes on the Eye when he is floating above you, and when he attempt to charges at you, try to dodge quickly, and when he comes up again continue throwing the the projectiles. In his second stage, he will charge at you several times. Try to dodge him and continue throwing the projectiles when he is away, and when it gets close use the spear (if you have it).
Eater of Worlds
In the Corruption, construct a straight line with platforms with approximately 25 blocks and repeat above 4 blocks until you have 5 lines of platforms and place some campfires, forming an arena. Have at least a Cactus/Ebonwood armor, some potions and some Shurikens/Throwing Knifes, Grenades and a spear (if available). Stay on the highest point of the arena and when he emerges from the ground, dodge quickly and keep throwing your projectiles (preferably grenades because they do more damage) or attacking him with the spear. When he divide himself, pay attention and continue with the same process.
Brain of Cthulu
Build an arena like you do with EoW (but this time in the Crimson), have at least a Cactus/Shadewood Armor, some potions, some Shurikens/Throwing Knifes, Grenades and a Rotted Fork (You should obtain it breaking a pulsating heart). In his first stage, just throw the projectiles at the Creepers when they're away, and when they get closer, use the rotted fork. In the second stage, the Brain will teleport around, charging at you. At this point, try to attempt where he will teleport and hit it with the Rotted Fork.
Build two straight line of platforms, one above the other with a campfire in the hive, have at least an Iron/Lead Armor, some potions, some Shurikens/Throwing Knifes and a Gold/Platinum bow with some Frostburn arrows. Before summoning the boss, jump in honey to get some regeneration. When the Queen Bee is charging at you, dodge quickly, and when she floats above you, stay on the lowest pointline of platforms of the arena you build (so the Minion Bees won't hit you because of the platforms) and keep throwing your projectiles/using the bow.
Build an arena, have at least Gold/Platinum armor (Shadow/Crimtane Armor is recommended if available), some potions, some Shurikens/Throwing Knifes, a Gold/Platinum Bow with some Frostburn Arrows (If you have Jester/Unholy arrows, you can use them) and a Spear/Trident/Rotted Fork (If available). When fighting, stay at the highest point of the arena, and when he swing his arms, jump down, and continue throwing the projectiles. When he start to spin his head, if you have a Spear use it, giving the max damage you can, because this is the time he is more "vulnerable".