You can choose to complete the game 3 times, once on each difficulty and the achievements will unlock, but there is a workaround. The achievements for completing the game on the different difficulties are reportedly glitchy and they may stack, but it isn't very likely. You should start by playing Hitman 2: Silent Assassin, as this is the earliest of the games, so you may as well play them in order.

The difficulty achievements in Contracts do stack and thus only 1 playthrough is required. This is entirely down to random chance and no fix is currently known. The difficulty achievements for Silent Assassin are reportedly glitchy and may or may not stack. The achievements for the 2 games, Silent Assassin and Contracts, are very similar to each other, so you will almost be earning the same achievements twice, but for 2 different games. The achievements from Blood Money can only be earned once however, so if you have already unlocked them, then you can only get the newly added 1000

You can still earn the achievements from Hitman: Blood Money, which is included in the trilogy, but has no new achievements of its own. This trilogy does not come with new achievements for Blood Money. Hitman Hd Trilogy, features 3 past Hitman games (Silent Assassin, Contracts and Bloodmoney) with a new set of achievements that can be earned through playing Silent Assassin and Contracts. A game that hardcore fans and casual gamers alike have been waiting for. Weapon collection achievements can also be glitchy, which ones and how many of them is seemingly random, No fix as of yet) Glitchy achievements: Yes (Difficulty achievements in Silent Assassin may or may not stack, depends on luck. Does difficulty affect achievements?: Yes Do cheat codes disable achievements?: No cheats Number of missable achievements: None (Mission Select) Minimum number of playthroughs needed: 4/2 (3 for Silent Assassin, but you can do just 1 full playthrough, provided that you start on professional, more details in step 1 and 1 playthrough for Contracts, as the difficulty achievements do stack) Approximate amount of time to 1000 : 35-50 Hours

Hitman 2 silent assassin guide