3 1/8 + 5 5/6 + 1 7/12

  31/8+55/6+17/12
= LCM of 8,6,12= 24
31/8*? =  /24 = 3*31/8 = 93/24
55/6*?=  /24 = 4*55/6= 220/24
17/12*? =  /24= 2*17/12= 34/24

93/24+220/24+34/24= 347/24 =  14 11/24 

ANS: 347/24
 14 11/24(Simplyfied)