O-methyltransferase; part of the gene cluster that mediates the biosynthesis of atranorin, a depside of polyketide origin that accumulates in the cortical or medullary layers of lichen thalli (PubMed:34154413). Atr3 methylates the carboxyl group of 4-O-demethylbarbatic acid to yield proatranorin I (PubMed:34154413). Atr3 is also able to methylate the atr2 product proatranorin III to produce the final compound atranorin (PubMed:34154413). The first step in the pathway is performed by the non-reducing polyketide synthase atr1 that produces 4-O-demethylbarbatic acid composed of two 3-methylorsellinic acid (3MOA) moieties. The pathway continues with the actions of the cytochrome P450 monooygenase atr2 that catalizes the oxidation of c-9 and the O-methyltransferase atr3 that performs the methylation of the carboxyl group to yield atranorin, via the proatranorin II and III intermediates if atr2 acts first, or the proatranorin I intermediate if atr3 acts first (PubMed:34154413). Secondary metabolite biosynthesis; terpenoid biosynthesis. Homodimer. Belongs to the class I-like SAM-binding methyltransferase superfamily.