I'm assuming you mean one boss at a time and not all three at once. If it is all three I'd say it's better to wait for better equipment first.
Having cleared that, I'd suggest you to fight the Destroyer first since it's the most easiest one. Make an arena if you haven't already, with at least three levels(ground + 2). Buff potions such a ironskin, endurance and wrath would greatly help.
As for weapons, I'm not sure about that clockwork assault rifle, I've never used it before. Onyx blaster would be great when used with crystal bullets. The best weapon would be Daedalus Stormbow combined with holy arrows, allowing you to defeat the Destroyer in a matter of seconds. Make sure you use the correct headgear for increased ranged damage.
Once you defeat the Destroyer, upgrade to hallowed armor set and fight the other two. Above mentioned weapons would be great against these two as well. Another great weapon would be the megashark, which can be obtained after you have defeated the Destroyer iirc.
For accessories: cross necklace, cobalt shield(or higher), charm of myths and a ranged emblem would be a good choice.
Hope I helped