Many historical forms of this character look more like an hourglass (𠄡) than the modern 五. Ponte Ryūrui says that its shape follows that of “a wooden receptacle equipped with two lids”, and could even be “an allegory of Heavens and Earth”. Wiktionary suggests that
五 was originally written as five horizontal lines, similar to 一, 二, 三, and the obsolete 亖 (“four”), but in common writing the lines would blend together. Thus, two lines were turned vertical and the right one was shortened, to form one stroke with the middle horizontal line.Whether or not this was the case, that’s how you can (remember how to) write 五: start with 三 and then add 二 rotated 90° (which could explain why two vertical strokes are unequal in length). In reality, however, the stroke order in 五 is quite different:
The Chinese hand sign for “five” is an open hand.
There is another hanzi with the same meaning, 伍, mainly used for financial purposes. This time it is easy to see 五 there.