To find out what fraction of the mural has been painted, we need to add the fractions that represent the amount of blue, red, and green paint used.
One fourth blue can be written as 1/4, two 8 red is equivalent to 1/4 (since 2/8 reduces to 1/4), and three 12 green is equivalent to 1/4 (since 3/12 reduces to 1/4).
Therefore, the fraction of the mural that has been painted is:
1/4 + 1/4 + 1/4 = 3/4
So, the students have painted 3/4 of the mural so far.