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Five words carry B second and R third, and not one of them is everyday vocabulary.
All five are obscure. ABRIM means full to the brim, ABRIN is a poison drawn from the rosary pea, and ABRAY is an archaic verb meaning to awaken.
So this is a pattern to read rather than guess at. If a puzzle puts B at two and R at three, the answer will be a word you half recognise at best.
I takes it in three of the five and A in two. That is the whole division, with ABRIM, ABRIN and ABRIS on one side and ABRAM and ABRAY on the other.
Confirming the fourth letter therefore cuts the group roughly in half, which is more than any other box manages here.
A toxic protein found in rosary pea seeds. It belongs to chemistry and toxicology writing rather than general use.
It is also an old adjective meaning auburn, which is why it appears here rather than being treated as a proper noun.
ABRIM is the most guessable, since its meaning follows straight from brim. The rest give you nothing to reason from.