先看一下COCA语料库类似的例句：

To make fake blood, mix together three parts corn syrup, one part water, corn starch, lots of red food coloring, drop of blue food coloring, drop of yellow food coloring.

Mix together one part glycerin and three parts water to make your fog juice.

Use three parts baking soda to every one part of tempura paint...

But the biggest reason is that Obama's campaign to date has been three parts dragging Romney down, two parts defending the President's own decisions and one part articulating an actual way forward.

由此可以看出，表示一个整体由哪几种物质构成，以及每种物质在整体中所占的比例为何，就直接在构成成分前用n part(s)表示各成分在总体中的比例。part和名词之间不用介词。

你的句子意思是，甘油三酯中所含脂肪酸和甘油的比例是3比1。