Well-formed, individual infiltrating glands (well formed gland – continuous lining and with lumina or space in the middle)(image A) & (image B).
Includes very large gland (pseudohyperplastic variant), as well as small but well formed gland (microacinar)(it's not about the size).
A good rule of thumb is that if you can mentally draw a circle around each gland without running into another gland, you're dealing with a pattern 3 (versus fused glands of pattern 4).
The difficult cases in needle biopsy are usually small focus of Gleason pattern 3, because they look the closest to benign glands.
In contrast to benign glands, carcinoma glands are usually small-caliber crowded and have rigid or "sharp" lumina.
Other nuclear, cytoplasmic and luminal features also important (as enumerated above).