In your inventory.hps file, add a combine callback to all three items. One for item A and B, one for B and C and one for C and A (so that you can use any part of each other). They all call the same callback.
In that callback, do an if-statement to make sure all three items are present in the player's inventory. Do something like
if(HasItem("ItemA") && HasItem("ItemB") && HasItem("ItemC")) {
//Remove and give items.
}
Inside there, you do what you want to happen when they use them. Most likely you want to remove the drill parts and give the player the complete drill item. If you wish you can add an else block that displays a message if they do not have all parts. Right below the if-statement:
else SetMessage("Message", "NotAllItems", 0);
The category here is Message and the entry is NotAllItems. Rename them if you wish.