From what I understood, the level is divided into 4(?) zones. You cannot enter zone B before you kill each enemy in zone A (there will be an invisible wall where the wooden boards are.) The three coins are located in zone A, B, and C (the last zone - D - is where you exit the level)
When I made this vid, I had no indication of where the coins were, apart from mortug telling me where to look (it was my very first playthrough), and I pretty much had to search for them. I didn't realise that you could just walk straight up one alley, and down the other. But as with mortug, don't feel like replaying this game

Besides, editing the vids is a bitch!