Sorry. Per my serving chart (my servings are 3"x1"x4"), the most number of servings you can get out of a 3 tier square cake is 133, unless you serve the top tier, and then you can get 154. That would be an 8-12-16.
To serve 170, you would need a 5 or 6 tier cake. a 6-8-10-12-14 will give 167 (not including the top tier) servings and a 4-6-8-19-12-14 wll give you 179 (not including the top tier).
The difference between the layers really depends on the design. If it's just a simple border, you can get away with a 2" difference. If there are flowers going all the way around the base of each tier, then a 4" difference is neccessary.
If you want to stick with a 3 tier cake, then you'll have to use kitchen cakes to fill out the rest of the servings. A 6-10-14 will give you 98 servings if you save the top tier. You would then need a full sheet (16"x24") kitchen cake to fill out the remaining servings (between the bridal cake and the kitchen cake you'll have 194 servings, so there will be some left over). Or you could do a 6-9-12 for the bridal cake (75 servings, so plus the full sheet kitchen cake you'll have 171 servings).