using SqlDataSource with parameter that is filled in other control

[Stu]

I have a GridView that pulls straight from a table. This uses
SqlDataSource1.
SqlDataSource2 grabs data from the same table, but only 1 row to fill a
DeatilsView, this SqlDataSource2 uses a ControlParameter to get the
SelectedValue from the initial GridView to isolate the 1 row.

My question is this: in the DetailsView, a column of data is returned
that I would like to use to send as a parameter to a 3rd SqlDataSource,
how do I do this?

I realize that I have to setup my 3rd SqlDataSource with a generic
<asparameter> object that I'll have to fill somewhere at runtime, but
where and how?

Do I use the Databound event on the DetailsView?

Any help would be appreciated, thanks!