There are multiple ways to wire 3-way switches and if you don't know which one was used, the simplest way to connect additional lights is to connect them in parallel with the existing ones. However, something tells me that you would not be asking the question if you knew exactly where the cable that goes to existing lights is, right?
<strong>We have to assume at this point that the power circuit feeding the kitchen lights is disconnected at the breaker panel. Always flip the breaker and then check if the power is indeed off.</strong>You would need to open both switches and see if there are more wires in one of them or they both have the same exact amount. Can be three - red, black and white (green or bare copper does not count) or five 2 x black, 2 x white and red.
Unless they both have the same five wires, you have to pull a cable from the new lights to the old and connect them in parallel - white to white, black to black. Otherwise neither of the switches by itself will have enough conductors to energize a new light in a 3-way arrangement.
If there are five colored conductors in each switch, you are not out of the woods yet either. The only way the new lights will work from a switch is when you pulled that new cable to the switch that has the wires to the old lights and not wires leading back to the panel. In other words, you will still be connecting the new lights in parallel to the old but only in this case such connection is possible inside of a switch box.
You can check which two-wire cable leads where (again, we are not counting ground wire here - green or bare copper) by measuring its resistance with a multimeter. The one leading to the panel will alternate between an open circuit and 20 Ohm (or less, depending on the power rating of the bulbs) when you flip the switch. The cable leading to the light will just stay at 20 Ohm regardless of the switch position. To make matters more interesting, LED or CFL bulbs would not measure that easily, so be sure to have at least one incandescent bulb.
Anyhow, without knowing much details about how your switches are wired, I'd say the best place to pull a cable from the new lights is into the rough-in box of the old lights. This way you know for sure that you can make it work.