SSS
+TTT
TTTR

There must be a 1 carried to the left-most column,
so we put a 1 above it:

1
 SSS
+TTT
TTTR

Therefore T can only be 1, so all the T's are 1's:

1
 SSS
+111
111R

If there weren't any other 1's to carry, the S's would have 
to be 0's. But the S's can't be 0, for SSS cant be 000, so 
there must also be 1's carried to the next two columns of 
digits too, so we put 1's above them too:

111
 SSS
+111
111R

So the only thing S can be is 9 so that when we add it to 1 
plus the 1 to carry we'll get 11.  So the S's are all 9's:

111
 999
+111
111R

And finally we see that the R must be 0:

111
 999
+111
1110

Edwin