They can be cleaned and stratified right away. After the first cold period the seeds will germinate and form roots. After the second cold period they will usually send up a single small leaf.

I have tried forcing them using a fridge and a growth chamber but didn't have much luck after the second cold treatment due to algae and mold forming around the pots. The challenge was keeping them from drying out for all that time and in hindsight a flat would have been better than individual pots. The success rate was much higher just planting them in the field and waiting.

Here is a link that might be useful: Trillium seed propagation