Firstly, yes they are taking advantage.
Secondly, stop relying on those multibuys to earn a bit of cash from your flatmates. It's hard to deal with money related things with others, especially with people who you are constantly near you, as arguments can easily start and things can get awkward.
Find a part time job or something instead, it will be hard to study at the same time but if you have to then just do it. Many people do.
Just buy the cheapest items available, if you're buying a multi buy, why don't you just keep the free item for yourself as it means you won't have to spend money on buying another one next time?