Well... let's not be so quick to condemn 3-d, recent MBA or not... there are a few packages out there designed to create 3-D plots and which allow you to "explore" the space by rotating etc. I don't think they would sell if they didn't have value.
But anyway...
Valori - what you might try doing is offer a slightly different solution to your user. I've had cases where I wanted to plot 3 dimensions but did something else with better effect. One in particular worked out great... Along the x axis I had day of week. Along the y axis I had time of day, and on the imaginary z axis I wanted performance statistics. Rather than creating a z axis, I did a surface chart in 3D, and used color to denote depth. In my case, red (hotter) colors were bad, blue (cooler) colors were good. I rotated the chart so it was viewed from straight overhead and the only way to read "depth" was with color, which worked out nice. I got my point across and even my boss' boss understood my charts.