Duncan, Malcolm, Donalbain, Banquo, Lennox, Macduff, Ross, and Angus arrive at Inverness. Duncan comments on the sweetness of the air. Banquo notes that martlets, a species of bird that usually nests in churches, have nested in the castle.
It is ironic that Duncan thinks the castle where he'll be murdered is beautiful. The beauty of the castle, and the way it seems blessed by nature with sweet air and nesting birds, also shows what Macbeth loses when he gives in to his ambition to commit the unnatural act of murder.
Lady Macbeth warmly greets the King and the thanes, though Macbeth is nowhere to be seen.
At this point, the planned murder weighs more on Macbeth than on Lady Macbeth.