When the argument is a Young tableau the shape is read off and the dimension depends only on that shape; the explicit standard tableau {{1,2},{3}} has shape {2,1}, so the irrep dimension is 2:
In[1]:=
TableauDimension
YoungTableau
[{{1,2},{3}}]
Out[1]=
2
A single-row tableau corresponds to the fully symmetric (trivial) irrep, which is one-dimensional:
In[1]:=
TableauDimension
YoungTableau
[{{1,2,3}}]
Out[1]=
1
A single-column tableau corresponds to the fully antisymmetric (sign) irrep, also one-dimensional:
In[1]:=
TableauDimension
YoungTableau
[{{1},{2},{3}}]
Out[1]=
1
Passing the bare partition gives the same result; no filling of boxes is needed: