Is my boss out of order?

It could be a lot of reasons, from what they have told you, maybe they don't need the extra help, or maybe they just don't feel you are performing up to par with others, I would just get a different job if I was you, most employers only seem to care about their needs, this is one that seems to think that, and they should either cut down the staff size so everyone gets more hours, or find a way to better utilize everyone, unfortunately they will keep doing this too you, if you are good at what you do, go to the boss and let them know you deserve more hours, if they still aren't willing to co-operate, let them know you are going to look for work elsewhere, giving you 0 shifts in 3 weeks time is completely absurd!