Trying to make a custom furnace

Hey, so I'm trying to make a custom furnace that has 1 item in an input slot, 2 more input slots, 1 with redstone in it and 1 with glowstone in it. Then a single output slot. I can't get more than 2 input slots to work, though. So as soon as I add the 3rd one and everything looks okay, when I open up the game and right click it, the game crashes. Nothing opens up. But right when I get rid of the code involving the 3rd input slot, everything works fine. I don't really understand what's going on. Maybe MCreator just doesn't support more than 2 input slots in a GUI? I don't really know. And the gui is linked to the block, the block inventory is enabled, etc. Still doesn't work.

Okay, second thing. When I do get it to work, I have the tick rate of the block on 100 so it takes a bit to smelt it. How can I add a progress bar / arrow like the vanilla furnaces have?

Thanks!