content bar + slider width

Hello, i just bought this amazing theme and i am a novice on css/wordpress. I am sorry if this Problem doesnt fit in here. I use the Theme as a 3-column Layout. The Logo-width is 1600px. I want to stretch the 3 Colums up to 1500px. 15% 70% 15% for the 3-columns for a total of 1500px. I am using a Child Theme and cant find the css part for strechting all 3-Columns. I am able to stretch the slider but the right column wont move to the right side so the slider is in the right column. It would be enough if the middle Column (Slider and articles) would stretch and the 1 and 3 column would stay on the width and just move to left/right. I hope you understand what i need. English isnt my motherlanguage. Thanks for your help. Martin