Okay, after looking at this for some time thinking it can't be possible, or that I'm missing some special physics of fuses
, here's my idea:
Spoiler:
We take 3 fuses and light them all at the same time.
We light fuse A from both ends. This should make the fuse burn in half the time, ie 2 minutes.
We have lit fuse B from one end only, but on the 2 minute mark, when A is burned, we light the other end of B too. The time remaining on B was two minutes. Now, with both ends lit, it will burn completely in one minute. We are now on our third minute.
We have lit fuse C from one end only, but on the 3 minutes mark, when B is completely burned, we light the other end of C too. The time remaining for C to burn completely would be 1 minute. Because it is burning from both ends, it will take half the time, so it will burn in half a minute.
When C is completely burned, we are at exactly 3 and a half minutes from when we started.