Boa tarde! Segue a resoluçao que eu achei pra fazer o display.
.item3 {
background: #84EEC1;
color: #041833;
grid-area: 1 / 3 / 3 / 3;
}
.item4 {
grid-column: 1 / 3;
grid-row: 2 / 2;
}
.item5 {
grid-column: 1 / 3;
grid-row: 3 / 3;
}
.item6 {
grid-column: 3 / 4;
grid-row: 3 / 3;
}