What Is The Difference Between Treacle, Molasses And Golden Syrup?

Treacle or black treacle and molasses as it is know in the USA are essentially the same thing. It is the syrup obtained at a stage of extraction from raw sugar cane. The dark syrup left at the end of the extraction process, after all the sugar has been taken out is called molasses in Britain and Black strap in the US.
Golden Syrup, a product little known in America is a light golden mixture patented by sugar refiners Tate and Lyle. It is a highly concentrated solution of sucrose, invert sugar ( a mix of fructose and glucose) and some added non sugars which give it is distinct flavour.
Treacle is a dark sweet syrup made during the process of refining sugar. All these products are used extensively in baking and confectionery, cakes puddings etc. Molasses, Black Strap and Treacle add colour as well as flavour to anything to which they might be added.

According to: www.ragus.co.uk products that require a smoother flavour, refiners syrup (partial invert) is blended with molasses (50:50 ratio) which produces the well-known product called black treacle."   So looks like "black treacle" is in fact a mixture of partial invert refiners syrup and molasses, whereas molasses is just pure molasses.
Golden syrup is pure refiners syrup (partial invert) see: www.ragus.co.uk

Black treacle is sometimes called '' dark molasses", but it's different from Blackstrap molasses.  Black treacle is sweeter and has a lighter taste than Blackstrap molasses - Blackstrap has a much stronger liquorice taste to me.  In cooking, I find black treacle slightly runnier.  There is a clear explanation on Wikipedia:

en.wikipedia.org molasses is made from young sugar cane. Sulphur dioxide, which acts as a preservative, is added during the sugar extraction process. Unsulphured molasses is made from mature sugar cane, which does not require treatment with sulphur. There are three grades of molasses: Mild, or first molasses; dark, or second molasses; and blackstrap. These grades may be sulphured or unsulphured.

To make molasses, the sugar cane plant is harvested and stripped of its leaves. Its juice is extracted from the canes, usually by crushing or mashing; it can also be removed by cutting. The juice is boiled to concentrate it, which promotes the crystallization of the sugar. The result of this first boiling and removal of the sugar crystals is first molasses, which has the highest sugar content because comparatively little sugar has been extracted from the source. Second molasses is created from a second boiling and sugar extraction, and has a slight bitter tinge to its taste.

The third boiling of the sugar syrup makes blackstrap molasses. The majority of sucrose from the original juice has been crystallized, but blackstrap molasses is still mostly sugar by calories.[2] However, unlike refined sugars, it contains significant amounts of vitamins and minerals. Blackstrap molasses is a source of calcium, magnesium, potassium, and iron; one tablespoon provides up to 20% of the daily value of each of those nutrients.[3][4] Blackstrap, often sold as a health supplement, is also used in the manufacture of cattle feed and for other industrial uses."