1)   -5/8 + 1/4 + 7/12 =  -5*3/8*3 + 1*6/4*6 + 7*2/12*2 =

= -15/24 + 6/24 + 14/24 = (-15+6+14)/24 = 5/24

2)   1/5 + (-1/7) + 2/9 = 1/5 - 1/7 +2/9 =

= 1*7*9/5*7*9  - 1*5*9/7*5*9 + 2*5*7/9*5*7 =

= 63/315 - 45/315 + 70/315 = (63-45+70)/315 = 88/315

3)  -1/8 +3/9 + (-5/36) =  -1/8 + 3/9 - 5/36 =

= -1*9/8*9 + 3*8 /9*8 - 5*2/36*2 = -9/72 + 24/72 - 10/72 =

= (-9+24-10)/72 = 5/72