What does Single channel means in RAM

Solution
Nah you won't destroy anything, they just won't work in dual channel which would boost your RAM speed by %15 if you bought match pair modules. If they are same voltage and brand they will work just fine, even if they're not they should work normally.

Proofy

Admirable
Nah you won't destroy anything, they just won't work in dual channel which would boost your RAM speed by %15 if you bought match pair modules. If they are same voltage and brand they will work just fine, even if they're not they should work normally.
 
Solution
That will work fine.

DUAL CHANNEL - this splits the same data between two equal sticks thus increasing the speed you communicate to the CPU by double. It may or may not help with performance depending on what CPU you own, what you are doing, and how fast the memory already is.

Putting 2GB + 1GB means you are forced into single channel. Basically you have 3GB total and it's not an issue at all unless you were losing performance by not being DUAL CHANNEL as discussed above.

I'm guessing it's a basic work PC so likely that's just fine.