If your sweaters measure 21 across you're wearing them too big. I'd probably order the 3a as a 39 or 40 so they do 19.5-20" p2p but have someone measure your chest again to be sure. Keep in mind I don't own inverallan but have other chunky knits I bought fitted variants of and each one will differ slightly.
Oh by the way this might as well be an interest check. I don't really need to sell this but because the length is a bit short on my 6'3 self it's only been worn twice and someone else could probably get more use from it... if you don't feel like waiting 9 months. Not to hijack or anything but just as an alternative. Size 38.