To solve this expression, we need to follow the order of operations, which is parentheses, exponents, multiplication and division (from left to right), and addition and subtraction (from left to right).

Let's break down the expression step by step:

1. 140 - [25 : (42 - 11) + 4]
First, solve the expression inside the parentheses:
42 - 11 = 31
Then, divide 25 by the result:
25 ÷ 31 = 0.8064516129032258 (approximately)
Finally, add 4 to the result:
0.8064516129032258 + 4 = 4.806451612903226 (approximately)
Subtract this result from 140:
140 - 4.806451612903226 = 135.19354838709677 (approximately)

2. 4.3 + [8 - (2 + 3)]
First, solve the expression inside the parentheses:
2 + 3 = 5
Then, subtract 5 from 8:
8 - 5 = 3
Finally, add 4.3 to the result:
4.3 + 3 = 7.3

3. 15 - {15 : [6 - (1 + 2)]}
First, solve the expression inside the parentheses:
1 + 2 = 3
Then, subtract 3 from 6:
6 - 3 = 3
Then, divide 15 by the result:
15 ÷ 3 = 5
Finally, subtract 5 from 15:
15 - 5 = 10

So, the final result is:
135.19354838709677 - 7.3 - 10 = 117.89354838709677 (approximately)