I've purchased a couple of packs of 3" Disc drive belts from Ebay recently.
From both suppliers, the Belts were too wide, they slip off the pulley as they don't sit right.
I solved the problem by cutting the belt along the length with a ruler and craft knife. This solved the problem.
I also temporarily tried a rubber band, but those slip and don't last as long, so not recommended.
In the future I need to find a supplier with a narrower drive belt.
At the moment, everything seems to be ok. Fingers Crossed. :)