getting atleast 7 in 3 tosses
(6,1,_) , (5,2,_) , (4,3,_), (1,1,5) , (1,2,4) , (1,3,3) , (1,4,2) , (2,2,3)
Now combination of (6,1,_)=6
(6,_,1)=6
(_,6,1)=6
(1,6,_)=6
(1,_,6)=6
(_,1,6)=6
Such combination also possible for (5,2) and (4,3)
Now for (1,1,5) = 3 combination
(1,2,4)=6 combination
(2,2,3)=3 combination
(1,3,3)=3 combination
Total combination 36+36+36+3+3+6+3 - 18(for repeated elements)=105
So, probability = 105/216