How to install ram 3gb in 4 slote

plug them in?
I'm not really sure what you want an answer to.

First of all, mixing different RAM that wasn't part of a kit is a hit or miss thing and result WILL vary.

As for how you put the stick into the PC, that first requires you to know what type of RAM you have, and what generation CPU/motherboard you also have.

If you don't know what I mean answer the following:

Is the RAM DDR2 or DDR3?

I all of the RAM the same type of DDR memory or do you have some DDR2 and some DDR3?
Because I'll let you know right now, you CANNOT mix different generation DDR with each other.

Also, what CPU is in the system that you want to put the RAM in?

Please provide actual CPU details.
We need to model.
Example:
Intel Core i5-2500K

Where the brand is Intel.
The family of CPU is Core i5.
And the model is 2500K.

Please give us the model of your CPU and the type of DDR you have.