How do I get through The Final Hurdle with 3 cubes?
First, put one cube in the slot that opens the first door
Then walk through the laser that opens the red doors
Next, block off the other laser with the two blocks you have
Then, walk past and take both blocks
Use one block to prop up the last door
Use the other block to jump over the laser that you previously blocked off
After you are past the laser, grab the block and use it to prop up the first door
Then grab the first block, and run back through, grabbing each block as you pass it.
Success!