We're doing fate points too.
On the original idea, optimally you would use two d8 dice numbered 0-7, and one d6 numbered 1-6 (standard). That would give you a range of 1-20, with a mean of 10.5. So, the same range as d20 but on a bell curve. With 3d6, you have a range of 3-18 so a lot of UM results on the tables are not merely going to be rare, they are impossible. E.g. you simply cannot fumble.
If you numbered the d6 5, 10, 15, 20, 25, 30, and the d8's 0, 5, 10, 15, 20, 25, 30, 35, you can use standard tables. Doesn't solve the math issue but does change the probabilities. You can't roll below 5 so you have to treat 5 as your fumble and open-ended down.