Nobody else has been told he’s coming, have they/ isnt he ?
We’d better not stay up too late tonight, didn’t we/ have we/ had we?
I suppose it’s time we called it a day, didn’t we/ isn’t it/ don’t I?

Nobody else has been told he’s coming, have they? (have they относится к nobody). We’d better not stay up too late tonight,  had we? (had we относится к We’d better = We had better переводится как нам не следует). I suppose it’s time we called it a day, isn’t it? (isn’t it относится к it’s time).