What strategies work well against the three hardmode bosses?

Personally, my loadout is as follows:

• Obsidian Shield
• Ranger Emblem
• Demon Wings
• Spectre Boots
• Philosophers Stone

I reforged my items to all have at least +3 defense each. Ranged damage might also be good. My main weapon at the time was a clockwork assault rifle equipped with either crystal or cursed bullets. The splash from crystal and the afterburn from cursed work rather well against bosses.

Preparing for battle:

• Set your spawn point to a bed in the Nurse's room, with an extremely easy exit out back into battle.
• Obtain several of every buff potion that may have even the most minor advantage in battle.
• Go to a relatively flat location outside your town, just far enough that monsters spawn, but not too far to where if you used your magic mirror that the boss would despawn.
• Wait at this location until the 'night music' starts. As soon as the music changes, you can summon bosses.
• Just before or immediately after summoning the boss, press 'B' to drink all your buff potions simultaneously.

During battle:

• If you are missing more than 100 health and you can drink a potion, do it immediately. The health potion cooldown is quite painful, and restoring yourself to full health is better than waiting for the cooldown to wear off when you're at low health.
• If any buff potions wear off, don't hesitate to press 'B' again. Potions are rather expendable, so wasting a couple potions is much better than risking dying and losing the boss battle.
• Use the gravitation potion to your advantage. It is much faster and more responsive than your wings, and does not have a limit to how high you can go. Fighting in the air is much easier than fighting on the ground, as you have 360 degrees of maneuverability.
• If you are stuck with low health and potion sickness, use your magic mirror to head back directly to the Nurse to heal up. You only have a few seconds to do this before the boss catches up to you, so be prepared to heal and immediately jump back into battle. Be sure to take the battle away from your town if the boss does catch up to you.

These are the strategies I used to defeat the bosses, hopefully they work for you as well!