Should we avoid eating in non-muslim restaurants completely ? As they cook non-halal products which may contaminate our own ordering. Or as perhaps, they do not wash their plates 3 times after it touched a najis food such as pork or non-halal meat for example.

Or on the contrary, can we eat in these kind of restaurants because we do not have certainty that our ordering becomes contaminated in the kitchen? And everything is tahir unless proven otherwise.

There is no need to wash the plates 3 times. One time is enough, except for Alcohol drinking glasses.
On the other hand, uncertainty is enough.