It may not be possible to get this to work with your given wiring but the fact that there are non-hot white wires is promising. Are there black wires wirenutted together in the box not connected to the switch?

No Ron, there aren't other black wires into the box.

You have to know which hot is the power source. If the power source is not in that switch box then it wont work.

(A and B on the Leviton 5245 are connected by default with a metal strip)

Run a wire from the power source hot to both a and b and break the metal strip between A and B. Run a wire from the connected whites to E. This should isolate the outlet from the 3 way circuit.

From what I found online it looks like this outlet is meant to be controlled by the 3 way circuit but you can remove that metal strip and isolate it.

How can I check which hot is the power source ?

It sounds like your switches are wired like this.

If you disconnect "a", and one light doesn't work, and you disconnect "b", and the other light doesn't work, then that switch box is wired like the right switch in the diagram, and there is no constant power to supply the receptacle. What you want to do would only work if it was wired like the left switch in the diagram.

In the attached picture you can see the actual configuration. On the Leviton 5245 sw2_1 and sw2_2 are connected together with a wirenut and i removed the break-off fin located between the black screws.

I tried to measure the voltage for each screw of SW2 using a multimeter, that's the results:

With light OFF (light switch off):
neutral-sw2_1 = 0V
neutral-sw2_2 = 0V
neutral-sw2_3 = 0V
neutral-sw2_4 = 120 V

With light ON (light switch on):
neutral-sw2_1 = 120V
neutral-sw2_2 = 120V
neutral-sw2_3 = 0V
neutral-sw2_4 = 120 V

Thank you!

Vith can i take power from the red traveler wire ? I mean, what's happen if i unplug the black cable from sw2_2 and connect sw2_2 with the red traveler ?

If you flip the toggle on switch 1 the other direction, you will find that your readings will be like this;

With light OFF (light switch off):

neutral-sw2_1 = 0V

neutral-sw2_2 = 0V

neutral-sw2_3 = 120 V

neutral-sw2_4 = 0V

With light ON (light switch on):

neutral-sw2_1 = 120V

neutral-sw2_2 = 120V

neutral-sw2_3 = 120 V

neutral-sw2_4 = 0V

The red traveler will no longer be hot, and the receptacle will still only work when the light is on.

Take the travelers off of switch 1 (wires 1&4), and test them for voltage. They will both be zero, and so will everything on switch 2. If the only power from one switch to the other goes through the switch, then the receptacle will only work when tge light is on.

You didn't accomplish anything by removing the break-off fin located between the black screws, and then tying them back together with wirenuts. The only reason to break the tab is if the switch and receptacles are on two separate circuits, or if the power and switch leg are in the same box, which you do not have according to the way the switch was originally wired.

Here is the current path of 3-way switches with the switches flipped in all the possible directions.

The red wire does not always stay hot.

You didn't accomplish anything by removing the break-off fin located
between the black screws, and then tying them back together with
wirenuts.

Because it wasnt done at the power source hot. Marco, you need to think three different animals here 1. the power source 2. the three way switch circuit & lights 3. the solo outlet

First task is to find the original hot. For you to keep both the 3 way switch and have a separate outlet, the outlet needs to be located in the box with the original hot OR a separate wire needs to be run to supply independent power to the outlet in the second box.

The 3 way switch wiring is its own animal and cannot include the outlet if you want the outlet to not be dependent on the switches. You can't use the traveler as a constant hot unless you get rid of the 3 way. If you got rid of the 3 way, you would use black for the single light switch and red for the outlet hot.