3D Assassin is a point and click adventure game.
Click and hold on room objects and inventory items for the verb disc. Use disc actions (look, talk, and use) to interact with the environment.
Click once on an inventory item to activate it as the game cursor.
Click an item cursor on objects and other inventory items to use. Click an arrow cursor to transition to a new room.
To clear an item cursor, double-click the mouse.
Good Luck!!!

Play 3DA - 3D Assassin

Walkthrough - Guide - Solution - Cheats
Get the plank from the top of the window on the left (use the hand icon 4 times)
Use the plank on the blinds of the window on the right
Open the guitar case
Get the rifle and the bullets
Combine the bullets with the rifle
Get the briefcase (I don't know why it is there, because it doesn't have any use)
Look at the toolbox
Look at the padlock
Try to use the padlock
Get the hair grip from Ms. Halloway's hair
Use the hair clip to open the padlock
Open the toolbox
Get the hammer
Use the hammer on the bottom board of the left window
Use the rifle on the left window
You can try shooting, but nothing will happen yet
Go back and use the hammer on the hole
Use the plank from your inventory to reach the camera in the hole
Use the camera on the left window and take a picture
Go back and open your inventory
Hold the mouse botton down on the picture and examine it
Note where the shining spot in the picture is
Use the rifle on the left window
Shoot the building where the shiny spot on the picture was
Done!

Source: EscapeGames24

Online Games point and click walkthrough Solution 3DA 3D Assassin