Hold the CTRL key pressed when dragging a clip from the the project panel, or use the , key from the source monitor to insert at the CTI or use . to overlay.
As a heads up. I did press the control and dragged my clip inbetween other clips already in the timeline and that worked. However when I wanted to drag the next clip, it brought forward the one I had just added and when I wanted to drag the 3rd clip in a row, the previous two clips came with it. I haven't tried the other two ways you mentioned yet.
If you're keeping the Control key depressed as you select each clip, you're adding to the selection, and therefore, adding the clips multiple times to the sequence. Select each clip individually, then press Control and drag to the timeline. Release, rinse, repeat.