Force the z-index of the first grid panel with this CSS style
[ng-grid=gridOptions1] .ngTopPanel {
z-index: 2;
}
A better approach (as suggested in comments) is to use a nth-child approach. extended to 3 items:
.gridStyle:first-child .ngTopPanel {
z-index: 3;
}
.gridStyle:nth-child(2) .ngTopPanel {
z-index: 2;
}