I agree with Rich. Best not to mix and match.
TBH, you may be better of getting that 2GB set, and installing it. But do away with the 1 you have now.
The reason I say this, youll be in dual channel mode which is more effecient than single.
I think, if you add the 3rd, it may well drop back to a single channel.
You also won't be mixing and matching
ALL the RAM will default to the slowest settings of the bunch.
(there many more sub timings besides the 4 main ones)
But, if the main 4 timings match, and the voltage is the same, it should work. I just wouldn't advise it as the possiblity of problems are higher.