whell i think i did find a solution for this problem. When you precompose all of the layers in one composition they dont become flat, insted of precomposing just the particles layers.they respond to the null 3d layer and now its ok.What it means is i have my 3d project set in one composition and i can trim it whitout some of my layers becoming flat.Tnx a lot people for your help and for posting some opinions.Regards ile