Since the tree is relatively near the corner of the yard, it would be better to create a bed below it that connects with the corner, rather than having that pitiful "strip" of grass to maintain between the two. As the tree grows and produces more shade, it's certain that it will only become more difficult for any grass nearby to grow well ... or at all.

Generally, in nature, any exposed ground does not remain bare but becomes populated with plants in a mere heartbeat. It's a good thing because the roots of plants hold soil in place and make things look better than if a space remained bare ground. Man generally thinks that a hodgepodge of weeds don't look good close up, but that tidy plantings do. By this reasoning, I'm claiming that a bed of groundcover below the tree will look superior to a lifeless bed of mulch ... especially after it weathers out.

I am not against borders around beds, but I'd avoid enclosing the tree area with any RAISED border of bricks or stones. (Especially, a tight, undersized area, like a 3' circle. It looks out of proportion to the scale of things around it.) Generally, such things around a tree are poorly installed -- waving and undulating -- and they will be thin of dimension, which all contribute to a cheap, junky look. Any border needs to be of a decently thick dimension (usually 8" wide min. for residential) and needs to be installed such that it is rock solid in appearance ... no waving or undulating. A flush inlaid mowing strip border is less likely to stand out poorly as is something raised up above ground level.