Flower:

Many clusters of 1 to 3 flowers arising from leaf axils and at the end of branching stems. Flowers are minute, about 1/16 inch across, have 3 white petals with pointed or blunt tips and stalks ¼ inch or longer. Rarely a flower has 4 petals. Stamens are white; there are as many stamens as petals.

Leaves and stem:

Leaves are consistently whorled in 4s. Leaflets on branching stems are mostly narrowly elliptic with blunt tips, up to about ¾ inch long and 1/8 inch wide, rough along the edges and main vein.

Leaflets on the main stem may be about twice as large as those on the branches. Stems are rough. Stems usually sprawl, becoming tangled in each other and in surrounding vegetation.

Fruit:

Fruit is a pair of tiny, smooth round pods, each containing a seed. The fruit is slightly larger than the flowers.

Notes:

There are only 2 species of bedstraw with mostly 3-petaled flowers, the other being Galium tinctorium (Stiff Marsh Bedstraw), which has more irregular leaves whorled in groups of 4, 5 or 6.  G. concinnum (Shining Bedstraw) also has tiny flowers and slender leaves whorled in 4s, but has 4-petaled flowers, leaves whorled in 4 or 6, and is found mostly in woodlands. G. labradoricum (Labrador Bedstraw) has leaves in 4s, but also has 4-petaled flowers. Of special note is species Galium brevipes, also known as Galium trifidum var. brevipes, is reportedly found in Minnesota, but this is a false record and neither the DNR nor the Bell Herbarium recognize the species' existence here. Interestingly, when researching G. brevipes to find out what its story was, the name popped up quickly enough but I could not find a reliable description nor a single photo published of the species, only a few botanical drawings.