I created a group option to present 3 choices for users and a macro to
setvalue to a text box for each choice to pass the parameters for my query.

My question is: how to clear the previous text boxes (or set them to Null)
if the user makes another choice after the previous ones?

My try didn't work:

[Forms]![Margin Dialog]![Frame17]=1      SetValue 0 to Text 26
[Forms]![Margin Dialog]![Frame17]=1      SetValue Null to Text28
[Forms]![Margin Dialog]![Frame17]=1      Setvalue Null to Text30
[Forms]![Margin Dialog]![Frame17]=2     SetValue 1 to Text 28
[Forms]![Margin Dialog]![Frame17]=2      SetValue Null to Text26
[Forms]![Margin Dialog]![Frame17]=3     SetValue Null to Text 26
[Forms]![Margin Dialog]![Frame17]=3     SetValue Null to Text28

Thanks,