The Destroyer
Night time is the only condition required for summoning. It can also spawn naturally as of 1.2 and spawns following the prompt 'You feel a quaking from deep underground...' or 'You feel vibrations from deep below...'.
The Twins
They have a chance of spawning when night falls once Hard Mode is enabled, displaying the message "This is going to be a terrible night..." at dusk.
Skeleton Prime
He can be summoned with the Mechanical Skull or has a very small chance of spawning at a random time. His approach is signalled by "You feel the air getting colder around you" or "The air is getting colder around you".
Plantera
After at least one of either The Destroyer, Skeletron Prime, or The Twins has been defeated, a pink bulb will appear somewhere in the Underground Jungle. Destroying this bulb, which requires a Pickaxe or a Drill will summon Plantera.
Golem
It is summoned by using a Lihzahrd Power Cell on the Lihzahrd Altar, which is found in the Lihzahrd.
quoted from the Terraria Wiki
All regular-mode bosses can still be summoned in Hardmode.
As you see, there's a tiny chance of every Hardmode boss spawning at night.
There is no standard order to kill bosses.
It depends on your armor, weaponry and (by my own experience) Mythril Armor is needed for any of them.
It's probably the best to do it like this:
It's recommended to have a good piercing weapon for Destroyer.
The Destroyer strategies page would help you.
After killing The Destroyer, you can kill The Twins very easily by using the The Megashark.
But Skeleton Prime is different from others. Hallowed armor is recommended to defeat it.
Best Answer
Killing the Wall of Flesh for the first time in any given world triggers hardmode, which is when the Corruption/Crimson and Hallow 'V' is generated. It is also the first step to the path to the Moon Lord boss. Though you can summon the mechanical bosses on any world (even pre-hardmode), the "jungle grows restless" message will not trigger unless you beat the mechanical bosses after the world is already in hardmode.
So in summary, you beat the bosses in reverse order - you should have killed the Wall of Flesh first, then the mechanical bosses. Now that you've got WoF beaten, you can beat the mechanical bosses again and the message will appear this time.