Oh, hey, missed that the first time through. I don't think Adamantite Armor is that hot in most instances. Oricalchum is better, if you have it, Titanium is a huge step up for this fight particularly because of the damage immunity. Just fish for crates (use the Sonar potion so you don't go through bait as fast) until you get enough bars. I don't recall exactly, but I think I started hardmode last time with about 150 wooden crates, and by the time I'd opened them all, had enough of those bizarre ores that I didn't even bother breaking an altar until I was good and ready to start fighting bosses.
Titanium and Adamantite are alternate ores, so unless you do fish a lot (and save the crates for hardmode) you'll only ever get one of them in a single world.
I don't know that you necessarily HAVE to break an altar, even. I think you can summon the bosses without, so you never have to deal with a boss when you aren't at your arena. Don't quote me on that. But obviously, fewer sources of corruption would be a plus, if corruption bothers you.
The summon items for the three mechanical bosses work without breaking an altar. I've also used them in non-hardmode worlds in previous patches, but not since 1.3 was released, not that I think the ability to do so has been patched away.
Anyway, to deal with the three mechanical bosses the very first time, you really should prepare an arena, a bunch of buff potions and the best weapon you can bring at the time. If you don't want to read a guide to the battles ([url=http://terraria.gamepedia.com/Guide:The_Twins_strategies]Twins[/url], [url=http://terraria.gamepedia.com/Guide:The_Destroyer_strategies]Destroyer[/url] and [url=http://terraria.gamepedia.com/Guide:Skeletron_Prime_strategies]Skeletron Prime[/url], then fighting them a couple of times to study their patterns is probably going to be a requirement.
Taking out the Destroyer will allow you to craft a Megashark, which will make fighting The Twins and Skeletron Prime much easier. Taking out The Twins will let you make an Optical Staff and/or a Magical Harp (depending on how many souls drop), both of which are useful against The Destroyer and Skeletron Prime. The Flamethrower is only really useful against The Destroyer, so defeating Skeletron Prime first is not as useful as defeating one of the other two first.