There's a prevailing misconception you cannot use different RAM sizes together or that you cannot mix RAM brands. Simply put, that's not true. ... The answer is Yes, you can mix RAM sticks and RAM sizes and even different RAM speeds—but mixing and matching RAM modules isn't the best for system performance.

Is it bad to have unbalanced RAM?

With uneven amounts, memory cannot be interleaved and has to be mapped first to one stick and then the other. A program that is accessing a large contiguous chunk of memory will find almost all of its access going to one stick and there will be no bandwidth combination.

Can I use 3 sticks of RAM?

To answer your question yes you can use three sticks of ram, however you will not get the maximum performance from it like you will from matched pairs in dual channel.