you can try use the Validate event for each textbox, and need a boolean variable to enable the command
Set the TabStop property for the textboxes to True and the TabIndex property:
Private Form_Load()
Text1.TabIndex=0
Text2.TabIndex=1
Text3.TabIndex=2
End Sub
Private Sub Text1_Validate(Cancel As Boolean)
If Text1.Text="" then Cancel=true
End Sub
Private Sub Text2_Validate(Cancel As Boolean)
If Text2.Text="" then Cancel=true
End Sub
Private Sub Text3_Validate(Cancel As Boolean)
If Text1.Text="" then Cancel=true
Command1.Enabled=True
End Sub
This doesn´t prevent syntax errors, just checks the TextBoxes aren´t empty. If you want to Know more check Validate Event for TextBox in MSDN