you can add just one of the empty ram slots but most motherboards are configured in dual channel. Its kind of like RAID 0 where if you put in 2 then the RAM has double the frequency. The problem is it only works in pairs of 2 so if you add a 3rd one you will get the extra RAM space, but they will run in single channel.
In simpler term, you will have more RAM, but it will be slower.
If you really feel like you need more RAM capacity, go for it because it won't feel that much slower, but if you just have extra and want to use it, I'd recommend waiting until you get another stick in a matching capacity.