Wantitall4moi-
I'd recommend you double-check your math.
Using your numbers, assume that in the games that don't hit "3", you hit 50%. That would be a combined record of 278-278-68. Laying-110, you'd be down 27.8 units.
If you paid -120 (buying the "3" for 10 cents)
You'd now be 346-278. Your EV would be:
+346 (1.0) - 276 (1.2) or +14.8 units. Paying that 10 cents made a 40-unit swing, juice included.
Buying off the 3 is worth an average of 24 cents. Buying onto the 3 is worth about 27 cents (turning a loss into a push is worth more than turning a push into a win, since avoiding a loss saves juice).
This is the reason all those books offering 10c half-points on the 3 went out of business... If you give me a wager, and give me 15 cents for free with point buying/selling, you're toast, even with 10c normal juice.
Another bit of basic strategy... The "7" is always worth paying 10 cents, on or off (I think off is worth 14 cents, on about 16, but that is from memory).