This should do it. Must be used in child theme stylesheet or Custom CSS plugin, because it needs to work on top of parent style.
/* ========== adjust sidebar begins ========== */
#content, #content .entry-content { width: 100%; }
#main { width: 72.595%; }
#tertiary img { max-width: 100%; height: auto; }
@media screen and (max-width: 1024px) {
#tertiary {
width: 23.775%;
}
}
@media screen and (max-width: 880px) {
#main {
width: 100%;
float: none;
overflow: hidden;
}
#tertiary {
width: 100%;
margin-top: 40px;
padding-top: 30px;
border-top: 1px solid #ddd;
}
}
#secondary { display: none; }
/* ========== adjust sidebar ends ========== */