I agree with the defensive midfielder but not the other two.
Just personally, I think Delph has been immense this season, we have only looked rocky because we lost Delph AND Mendy at the same time, which is very unlikely to happen again, so I am very happy with those two.
I would say another striker/goal scoring winger is necessary if we ALWAYS use 4-3-3 because with 3 forwards we start to look a little thin up front if we have an injury. BUT, I think Pep will rotate between 4-3-3 and 3-5-2, and will probably favour 3-5-2 most of the time because of Mendy and Walker's ability to dominate the wings. This formation requires only 2 forwards, which we have alot of, buying a 5th forward when we'd play with 2 most of the time is a bad idea.
It basically boils down to which formation you think will be used most often, I think if it's 3-5-2 then 1 buy at most is needed.