Is there any such thing as a stock shaft material?

[Clive S]

Not sure about 3m long though - there may be a wall in the way!

Drill a hole in the wall of your workshop standard practise.

[magicniner]

Can you do it the way they used to in industry where everything fits on the shaft rather than having to machine the shaft to accept something?
If you buy shafting rather than round bar the OD will not require machining down to accept bearings.
Taper lock couplings could provide a way to securely fit ends on a length of stock shafting with pillow blocks (if and where required) to support the free length.
Everyone's poking around in the dark because there isn't enough requirement info in the question for a definitive answer,
Regards,
Nick