When you complete house of the dead 3 on any difficulty you get the choice to play in very easy mode or very hard mode!
The maxximum credits you can get are 9 and maximum lives per credit to start with is five but can be made bigger by saving teammate or getting bonus lives.
To get bonus lives in 2 save hostages and they will give you some!
To unlock a special minigame attempt house of the dead 2 original mode and you may unlock it when you get past a certain point!
After defeating the Lake Monster (the one with a spear and piranhas) a second time (this time he returns when you are driving on the bridge) you will drive up towards a few trucks blocking your way. After stopping to kill the surrounding Zombies, the trunk to one of the trucks will open. Inside will be a power up that can range from infinite credits (if this is your first time) to 10 (if this is your 2 or 3rd time). Shoot it, and it will become available for use as a power up in Original Mode.
While some may consider Extreme mode to be a difficulty setting, it is NOT. It is a completely separate option/mode that you can select in The House of the Dead III menu. In order to unlock it, you must complete The House of the Dead II + III on any difficulty setting. House of the Dead II can be completed on original mode to do this, but you must complete III on Arcade Mode (what other mode is there?). Until you have unlocked this, it will show up as "???" in The House of the Dead III menu.
In Extreme Mode, you go through the game the same way in Arcade Mode, but with various game play changes. Your bullet blast radius is MUCH smaller, and you can only hit enemies in the weak spot to hurt them. Also, enemies attack much quicker. On the upside, you have faster reloading with the shotgun and a new melee attack is available. The amount of credits/lives you have set in the options menu DOES effect how many credits/lives you have in Extreme Mode, as it does in Arcade Mode.
There is no Original Mode, however, for The House of the Dead III like there is in The House of the Dead II.