I was planning to make a verb that when you combine three items you get a new item.
Something like:
combine #object1# #object2# #object3#
remove object 1, 2, 3
move object "object4" to player
---
Is that possible to make? If it is then please post an answer!

yes, it is. Look at the appended file
@Alex: Could you look at this file, too? If you start the game, you get the message:
"Error running script: Error compiling expression 'game.objectdescription': RootExpressionElement: Cannot convert type 'Object' to expression result of 'Boolean'" Why that?

I don't see any error message...?

strange! Today I dont get the message. Yesterday I reloaded the file in Q5 several times but I did not restart Q5. Perhaps Q5 isn't reseted completly when restarting a game.

Well, I copied all the things you put in the file, but it doesn't work. Does it matter if there is a space in the name, like
"metal parts" or does it have to be "metalparts"?

Could be. The function deletes all spaces in the list

To keep typing for the user simple ... I'd probably make an object like a workbench ... and then set the 'use' script on the workbench so if the player had all three items in their inventory, then it would remove them and give the player the new item. Its also an opportunity to help the player figure out what they need ... if they try to use the workbench, but don't have the items it could say "You need an object1, object2, and object 3 to use this workbench."