Using standard deviations to group participants

I have scores from 83 participants, and would like to group them in to high, medium and low. The data is normally distributed.

I thought that I could allocate to groups using standard deviations, for example, 1.5 sd above is high, 1.5 below is low.

But this is where I get stuck.

If my mean is 49.17, and the sd 33.61, then what would my cut off scores be for each group?

I thought (33.61/2) + 33.61 to get 1.5 sd = 50.42, so my cut off would be 49.17 + 50.42 = 99.59.

But is that incorrect? Only 4 of my participants scored over 99.59.

Any help gratefully received! Thanks.