okay, got help on math tutoring site for confirmation/clarification. this is what he said.
You can get a rough estimate by finding first the average point total of any one card and multiplying it by 3: The total number of points in the deck is
4(1+2+3+....+9+10+10+10+10)=340, so the average per card is 340/52=6.54.
For three cards drawn from three complete decks, you would have on average 3(6.54)=19.6 points.
However, your three cards come from the same deck, and this where it gets tricky. Drawing the first card will effect the average points of the remaining 51 cards: for example, if you drew a king (of value 10) first, then the average point number for the second card will be lower than 6.54 (it would be (340-10)/51=6.47).
To find the exact answer, you would have to go through all possibilities ( (king, 7, 8), (ace,5,10), etc.) and calculate the value, then find the average. I suspect it would still be close to 19.6.
Looks like O/U 19.5 is about right. So I'm going with that. seems like a fair bet to me. Thanks everyone.