It can be done but it is usually not easy. I am not a licensed electrician and I am not trying to drum up business for an electrician but unless you are well versed with wiring, running wire, knowing how many splices can go in a box, and determining which wires are which, I highly recommend that you hire a good electrician for this job. There are 6 ways to hook 3 wires onto each three way switch. Trial and error doesn't work. I have attached a schematic for your reference.
You need to turn off power and remove the ex. Switch. These two wires are now the "power in" and "to light" conductors. One goes to the common screw on the new switch and one splices to a wire coming from the second switch (discussed more later).
You need to run a 3 conductor cable from the logic. Of the ex. Sw. To the logic. Of the new sw.
If you look at this schematic, you do not need the three wires shown going to the light. You already have those in the ex. Box. But, you need one of your new wires (in the three cond. Cable) namely, the black one on the right in the diagram, to take the place of the wire shown in the diagram going to the light. That wire will splice to the wire you removed from the old switch.
The new 3 way switches will hopefully have a sketch included so you can tell which screw is which. The pivot points shown on the sketch as a black ball are called commons. The other two screws are for travelers.
If you get it all hooked up and it works partially but not perfect, you probably have to reverse the travelers on one switch (not the common).
I suggest that you read this over and look at the diagram. If you can follow it, maybe give it a try. If you don't follow, don't do it. I have seen people sitting on the floor holding their head in their hands, after trying for a few hours to get it right. There is only one way that will work.
http://ezdiyelectricity.com/images/wiringdiagrams/switches/3-way-switch-wiring-diagram.jpg
You need to turn off power and remove the ex. Switch. These two wires are now the "power in" and "to light" conductors. One goes to the common screw on the new switch and one splices to a wire coming from the second switch (discussed more later).
You need to run a 3 conductor cable from the logic. Of the ex. Sw. To the logic. Of the new sw.
If you look at this schematic, you do not need the three wires shown going to the light. You already have those in the ex. Box. But, you need one of your new wires (in the three cond. Cable) namely, the black one on the right in the diagram, to take the place of the wire shown in the diagram going to the light. That wire will splice to the wire you removed from the old switch.
The new 3 way switches will hopefully have a sketch included so you can tell which screw is which. The pivot points shown on the sketch as a black ball are called commons. The other two screws are for travelers.
If you get it all hooked up and it works partially but not perfect, you probably have to reverse the travelers on one switch (not the common).
I suggest that you read this over and look at the diagram. If you can follow it, maybe give it a try. If you don't follow, don't do it. I have seen people sitting on the floor holding their head in their hands, after trying for a few hours to get it right. There is only one way that will work.
http://ezdiyelectricity.com/images/wiringdiagrams/switches/3-way-switch-wiring-diagram.jpg