I have built a soffit behind some cabinets and want to install 3 recessed lights, I have a constant 12/2 coming out of the wall and a 12/2 that is on a switch both went to exsisting light fixtures before the remodel, how do I wire this?

Are you saying that you have power coming to the first light (previous fixture) and a 12-2 cable to the switch?

There is some more information that we need.

1) Are the recessed lights IC or Non IC listed light fixtures?

2) Are the light fixtures listed for pass-through wiring?

3) will the circuit be a 15 amp circuit or a 20 amp circuit?

4) Is there adequate space in the soffit to disperse the heat?

In general, if the supply source is in the ceililng, then you must use a switch loop. You do this by connecting the constant Hot (Black) conductor from the panel to the White (retask this conductor by placing black electrical around each end or using a black sharpie) going to the switch.

The Black coming back from the switch will connect to the fixture blacks. The white from the supply will connect to the fixture whites.

I didn't un hook the previous fixtures but there is a 12/2 coming from a swith and a 12/2 constant coming out of the wall in the same spot. That I do know somehow was hooked to the previous fixtures. Thank you.

Presumably you know which is the constant hot cable and which is the cable going to the switch. Put a piece of black tape on the white going to the switch to mark it hot. Connect the black of the constant hot to the white (with the black tape on it) going to the switch . Connect the white of the constant hot cable to the white of the light and to the white of the cable going to next light. Connect the black of the cable going to the switch to the black of the light and to the black going to the next light.