$ perl -E '
> $_ = q{Bla class="x1" class="x2" class="x3" Blabla};
> say;
> s{" class="([^"]+)}{ $1}g;
> say;'
Bla class="x1" class="x2" class="x3" Blabla
Bla class="x1 x2 x3" Blabla