Why do you think Liverpool is behind Manchester City and Arsenal. You meantion Liverpool's subs as really weak yet the club still got the likes of Nunez, Mac Allister, Diaz among others in its substitutes some of which may not perform like the first level team but will be able to turn up and represent when the need always arises. What's confusing about your statement is that Liverpool is behind Manchester City and Arsenal when yet Liverpool are the leading club in the table standing right now.
I also disagree if it is said that Liverpool is behind Arsenal because, basically, Liverpool is still a better team than Arsenal. I personally do not care about their position in the standings but I think it is clear that Arsenal is still not strong enough in the competition for the title. Indeed,in the last two seasons Arsenal has become a better team for its competition in the title, but in reality Manchester City's dominance is still very high. Thus, it would be more appropriate to say that Arsenal is behind Manchester City and Liverpool, because in reality Arsenal often fails to maintain good and consistent performance, and that is also what makes Arsenal still fail in the trophy race from season to season lately.