I seen this problem.

Try:
Me.Sfrm_3.SourceObject = form_3
Me.Sfrm_3.LinkMasterFields = ""
Me.Sfrm_3.LinkChildFields = ""

So, right after you assing the form object, kill the link master/child...

I also found that if I modify the sub-forms sql direct, then again, I had to
go:

  Me.subTourFriends.Form.frmTourBuses.Form.RecordSource = strSql
  Me.subTourFriends.Form.frmTourBuses.LinkChildFields = ""
  Me.subTourFriends.Form.frmTourBuses.LinkMasterFields = ""

It's been awhile since I've done this, but if my memory serves me correct
the problem was that if you have a relationship setup in the tables, MS
access tries to do you a favor and sets the link master child for you based
on those relationships (and that's not necessary what you want).

In my above example, note how I had to kill master/ child settings **after**
I set that SQL.

So, if the 1st above suggestion of clearing out the link master don't work,
then:

You could try to ensure the sub forms data source is
NOT already set (leave it blank).

Thus, you go:

Me.Sfrm_3.SourceObject = form_3
me.Sfrm_3.Form.RecordSource = "your sql for form goes here"
Me.Sfrm_3.LinkMasterFields = Link_2_Parent
Me.Sfrm_3.LinkChildFields = Link_2_Child

I'm quite sure the above (last) suggestion will work with a blank
datasource. (not exactly a pretty work around, but I think it will solve
this And tell someone else jumps into this thread with a better suggestion).