Specify width of 3 chars for an html input text

Use the ch metric.

width: 3ch;

Make sure your box-sizing property is set to content-box if you're changing left and/or right padding.

Not reliably, since l and W have very different widths.

However, if you set the font to monospace, that helps. Then, you should be able to set size="3" on the input element, and in theory it should be exactly three characters wide.