Only three mistakes?

Here goes

1 (and absolute FIRST) fail to understand or provide for a ‘foul wind’.

Hence, lack both insurance or do not bother to save against a down turn.

2 fail to prioritise / budget their spending and almost invariably do not budget to save.

3 fail to understand how money borrowed has to be repaid.

I could go on for pages

Other popular monetary errors are

Fail to recognize how ‘return’ is a function of ‘risk’ (add most of the professional bankers into this category)

Completely ignore the adage "if it sounds too good to be true, it probably is"

Get drawn along on the wave of popularity or unpopularity (= being cyclical instead of counter-cyclical)