We can use across(everything()) in reframe.

my_data |> 
  rowwise() |> 
  reframe(across(everything()))

# A tibble: 4 × 3
     C1    C2    C3
  <dbl> <dbl> <dbl>
1     1     5     9
2     2     6    10
3     3     7    11
4     4     8    12