First, substitute the values of a and b into the expression:

3(2(5/12)-3(-4(3(3/12)+5/12))

Next, simplify the expression according to the order of operations (parentheses first, then multiplication and division from left to right):

3(10/12 - 3(-4(9/12)+5/12))
3(10/12 - 3(-4(3/4)+5/12))
3(10/12 - 3(-3+5/12))
3(10/12 - 3(-3+5/12))

Next, continue simplifying:

3(10/12 - 3(-3+5/12))
3(10/12 - 3(-3+5/12))
3(10/12 - 3(-33/12 + 5/12))
3(10/12 - 3(-28/12))
3(10/12 +84/12)
3(94/12)
3(94/12)
94/4 = 23.5

So, the value of the expression when a=5/12 and b=3 is 23.5.