Recently Steve and I thought about making a Strong Belgian Ale, and we reviewed some literature that mentioned tertiary fermenting, or three stage fermenting.

Has anyone experimented with this before

You can probably pull this one off with a single stage fermentation. A lot of literature likes to make brewing beer at home very complicated.

Sometimes people talk about bottle conditioning as a "secondary fermentation in the bottle". That might be the tertiary fermentation talk. Like if you did a normal secondary fermentation, then the tertiary fermentation would be in the bottle.

Another reason I could think of for tertiary fermentation would be incremental feeding of the yeast. Some say that adding sugars at various stages of fermentation is somehow supposed to make it easier for the yeast to finish their job. I am not sure I believe in incremental feeding, but this might be considered three-stage fermentation.

One more possible idea is that you could develop complex yeast flavor and aroma profiles in the beer by using multiple strains of yeast, and that could be done as multiple fermentations.

Final thought: if this is a pale beer, then you could maybe improve beer clarity with a third fermentation stage. But then you might be left with little yeast in suspension, which could complicate bottle conditioning in a high alcohol beer like a Strong Belgian.