Yep you could do all three at once. Ok so when you want to edit layers the first part is finding the right bit: this is usually something called a function or something starting with the word 'set' which is called a property. To do this you open the layer all the other layers depend on. It's called Core2 and you can find it at http://www.dreamwidth.org/customize/advanced/layersource?id=550&fmt=html.

A simple search for 'posting in' will lead you to:

property string text_posting_in {
des = "Text when user is posting in a community";
example = " posting in ";
}

and later this:

set text_posting_in = " posting in ";

What you wanna do for #2 and blank out this bit by changing it to this:

set text_posting_in = "";

Pretty easy so far, right? To do this you need to create a custom theme layer at http://www.dreamwidth.org/customize/advanced/layers but I think you already have one, correct? In this case, you only to add the line I mentioned above.

#1 and #3 are a bit more complicated. To find the bit you need to edit search for the more precise CSS class: entry-poster. At some point you'll find this bit:

# Prints the name of the account which posted the entry
function Entry::print_poster {
    var Page p = get_page();
    var string emptyclass = ((($p isa RecentPage and not $p isa FriendsPage) or $p isa MonthPage or $p isa DayPage) and $p.journal.journal_type != "C") ? "empty" : "";
    print safe "<span class=\"poster entry-poster $emptyclass\">";
    if ($p isa FriendsPage or $p isa EntryPage or $p isa ReplyPage) {
        $this.poster->print();
        if (not $this.poster->equals($this.journal)) {
            print $*text_posting_in;
            $this.journal->print();
        }
    }
    else {
        if (not $this.poster->equals($this.journal)) {
          $this.poster->print();
        }
    }
    print safe "</span>";
}

Now it's just a matter of reading the code. You can see where the SPANs are. You can guess that $this.poster->print(); prints the poster name whereas $this.journal->print(); prints the journal name. You can also see your by-now-familiar text_posting_in property (hmm seems you could have done all three here then!).

Now how to change it?

$this.journal->print(); is within an if thingie with brackets so to move it before $this.poster->print(); you have to move the whole thing like this:

# Prints the name of the account which posted the entry
function Entry::print_poster {
    var Page p = get_page();
    var string emptyclass = ((($p isa RecentPage and not $p isa FriendsPage) or $p isa MonthPage or $p isa DayPage) and $p.journal.journal_type != "C") ? "empty" : "";
    print safe "<span class=\"poster entry-poster $emptyclass\">";
    if ($p isa FriendsPage or $p isa EntryPage or $p isa ReplyPage) {
        if (not $this.poster->equals($this.journal)) {
            print $*text_posting_in;
            $this.journal->print();
        }
        $this.poster->print();

    }
    else {
        if (not $this.poster->equals($this.journal)) {
          $this.poster->print();
        }
    }
    print safe "</span>";
}

This takes care of #1.

You can also remove print $*text_posting_in;
This takes care of #2 (and you don't need the set line from before then).

Now #3 is a bit trickier because all of the conditional (if) statements. You gotta make sure that the SPANs are only printed if the element exists (there isn't always a community name or even a poster name) and that they're closed in the right place or you'll mess up your whole layout. I think it gives you this:

# Prints the name of the account which posted the entry
function Entry::print_poster {
    var Page p = get_page();
    var string emptyclass = ((($p isa RecentPage and not $p isa FriendsPage) or $p isa MonthPage or $p isa DayPage) and $p.journal.journal_type != "C") ? "empty" : "";

    if ($p isa FriendsPage or $p isa EntryPage or $p isa ReplyPage) {
        if (not $this.poster->equals($this.journal)) {
            print safe "<span class=\"poster\">";
            $this.journal->print();
            print safe "</span>";
        }
        print safe "<span class=\"entry-poster $emptyclass\">";
        $this.poster->print();
        print safe "</span>";
    }
    else {
        if (not $this.poster->equals($this.journal)) {
            print safe "<span class=\"entry-poster $emptyclass\">";
            $this.poster->print();
            print safe "</span>";
        }
    }
}

It might not be necessary to close the span both times but I'd rather do it that way to be on the safe side.