As there'll be little torque on the connection just some shrink sleeving on the shaft to bulk it up to 3mm should do the trick.
I did think about this before and discounted it because some of the shafts have on/off switches as well and they do have a fairly high torque requirement and also if I purchase these 3mm knobs and it doesn't work , I'll have wasted a reasonable chunk of cash as these aren't cheap and I really need 6 of them to match up.
I could end up looking around for a scope beyond hope and rob knobs from that if all else fails.
I have asked for a P&D on 6 knobs already from Foremost Electronics, lets see what they come back with. If reasonable, they are only a few miles from, I could collect them.