I have a question on Arthur's heraldry. Why is it three crowns, rather than, say, a dragon rampant?

I have a question on Arthur's heraldry. Why is it three crowns, rather than, say, a dragon rampant?

1. Because he ruled over three regions: Logres, Cambria and Albion/Albany
And if that doesn't quite make sense, then you are not reading the sources that said those were all his kingdoms
2. The heraldic system was more flexible than many books imply, especially in its early stages
3. An alternative arms has 7 crowns for him

I always thought that that's not really his personal heraldry, but the heraldry of the High King of Britain. Which may well be tree (or seven) crowns.

When Arthur becomes High King, no one, apart from Merlin and possibly Ector, know that he is the son of Uther, so he wouldn't use his heraldry anyway. According to PGC, his parentage is revealed first in 512 ("late February", actually).