I agree with Jazz. With a slot drill you'll need a couple of fixed steadies either side of the cutter and you would have to traverse a 6m bed, to machine a 3m length, which would have to be extremely rigid over its full length.

With plasma the work holding is much simpler, the workpiece is static in X supported on some v-rollers which are easy to fabricate, the lightweight plasma head moves in X along a 3m rail, probably using rack and pinion drive, so relatively easy and cheap to build.