Currently, you have to loop through the cells in the column, and format the string appropriately.
for(int row = 1; <= grid.RowCount; ++row)
grid[row, 17].Text = string.Format("= A{0} + 3", row);
In the next release (do not know when - if you want a private build, you can submit a Direct Trac incident and request one), there will be a convention if you would use row 0 in a formula, it will be treated as a wild card that can be used to enter the formula into the grid.ColStyles as you tried above. In that case, the grid will swap out the zero for current row as it parses the expression.