I'd like to know if masturbating with his/her boyfriend/girlfriend, during a sex cam session for example, is a sin ? Is showing one's private parts to his/her bf/gf is a sin? Does it take part in breaking the 3rd precept ?
The five precepts define ways that do not harm others. Forms of unconventional sex with a permissible (non-prohibited) partner do not fall under the 3rd precept. However, they may be unwholesome & stupid behaviors that harm oneself, either in the short or long term (such as cultivating unbeneficial & dangerous mental tendencies, habits & addictions).
There is no "sin" in Buddhism. Every act is evaluated on the basis of whether it leads to Peace or to Trouble, and in what degree. Any activity that feeds and cultivates Lust leads to Trouble, in a strong degree.
This is because Lust is what's known as "blinding affect", it makes one's mind blind. Blinded by lust you no longer know what's good / what's bad for you or for others, because you are so focused on achieving your desire that you have tunnel vision, you don't think.
Also, lust leads to objectification of the person which you desire. You no longer see them in all their complex inter-being with the rest of the world, you see a Thing which you want. When you are in this mode, you can easily harm the other person with insensitive acts.
Finally, lust leads to "subjectification" of you. When you project yourself onto the future and fantasize about being with the object of your lust, you cement an artificial image of "I" - which, as any fantasy, leads to conflict with the real world.
So no matter how you look at it, cultivating Lust leads to trouble for you and others. Will camsex not reenforce your lust and desire towards that other person? I don't think so.