What about digit '3' to be more distinguishable from cyrillic letter 'З'

[communist1917]

First — digit '3'. Second — cyrillic letter 'З'.

Same in PT Sans

[atheistray]

I think it's more of a stylistic choice. PT sans is designed by Russian designers for Cyrillic support, so they paid extra attention on distinguishing those two letters. But in most typefaces, even those designed for user interface such as segoe UI, Verdana, those two letters look very similar.