Safety of 3-Way Lightbulb That Exceeds Max Wattage Only On Highest Setting
No.

A three way bulb has two separate filaments that are lit separately by two separate contacts on the bottom of the bulb. The three way socket allows each of these filaments to be lit separately or together.

A 50-100-150 watt bulb has a 50 watt filament and a 100 watt filament. It draws 150 watts only when both filaments are lit at the same time.

If you put a three way bulb in a conventional socket, only the center contact will be lit. In your case, that would be the 100 watt filament. This does not exceed the max rating and you are fine.