The whole mace avril is "roughly broken" into pieces called blades. One blade of mace is supposed to be enough to flavor a meal of four to six portions. One teaspoon ground mace should equal one tablespoon mace blades. A blade of mace is roughly equivalent to a half a teaspoon of ground mace when substituting in recipes.
So, a blade of mace is a "roughly broken" piece of the whole mace. It is likely a significant portion, not a tiny piece, if one blade is supposed to flavor a whole dish? It is also likely to be less than a tablespoon (as one tbs is mentioned in plural), can be roughly equivalent to a half a teaspoon of mace - at a tsp per tbs conversion, perhaps a teaspoon and a half of cracked mace pieces, if they're on the larger side, of course - and less, a teaspoon or maybe under three-quarters of a teaspoon, for tiny cracked pieces since they'll pack down much tighter than larger loose curls.
The first picture below is supposed to be of very good quality mace blades, very photogenic, so likely to be of a good, standard, size. This can be compared to the second photo, of a whole mace husk. And taking into account that a recipe might use one blade for a whole dish as mentioned earlier, or might call for three blades as the OP mentioned (the difference between the amount of spice for a background vs primary flavor, perhaps), I'm thinking a blade is a significant chunk of the mace husk, but not the whole thing.
At a rough guess, maybe a blade is supposed to be a third to a fifth of a whole mace.
This is kinda a big difference, but that might play out in recipes which mention something like one large or two small blades. Of course, one can always season to taste, and add more or less as the dish requires - but I think grabbing a third to a fifth of a whole mace husk, or about a teaspoon of smaller mace pieces per blade, and adjusting from there might be a reasonable starting point.