[Russpig]

Geese have started to lay, I’ve had 3 eggs in the last 5 days.

I’ve lifted the eggs but I leave 1 dummy egg in the nest.
I’d love one to go broody, am I doing right lifting the eggs to prevent damage and leave the dummy egg until I have a broody, then put them under her?

[Penninehillbilly]

Yes, That's what I would do, don't forget to turn the eggs you are storing so the yolk doesn't settle.

[Russpig]

Great, and yea I’m turning them morning and evening.

Roughly how many eggs will be laid before sitting?

[Richmond]

Depends entirely on the goose. I have had geese go broody after laying only 5 eggs, and then others who lay over a dozen. However,  I would not encourage broodiness just yet. Goslings hatched in mid-late May get the benefits of the warmer weather and better grass.
My method is to remove all eggs laid until at least the beginning of April, then start leaving the eggs laid in one particular nest and DO NOT MOVE. Once that nest is being covered by the same goose pretty much all day long erect a barrier between her and the rest of the flock so no more fresh eggs are being laid into that nest and she gets peace and quiet to brood.
As you know goslings can take up to 32 days to hatch so it's a bit of a wait but I just leave them to it. They are not as easy as broody hens and best left alone as they can give you a real bashing if they want to and can break the eggs just trying to defend the nest. People are often nervous about ganders but a broody goose is far more ferocious! Give them a bucket of water nearby and offer food twice daily (remove if not eaten) and that will be all they need. If you have the space give them their own personal bath. They won't use it very often but maybe once a week they will appreciate a splash about.
When goslings hatch after about 24 hours place a bowl of waterfowl grower pellet close by so they can discover it (you will have to approach the nest very carefully not to agitate mum).  The goose will teach them everything else they need to know. 

[Penninehillbilly]

Great advice by richmond, especially how ferocious broody geese can be! and they can knock their eggs out of the best while defending it. Also another thought, don't try and put too many under her, some may get pushed to the side or out of the nest and not be kept warm enough.