Hi , @vicsaisan
Thanks for your response!
According to your description , you use this code in Combo box 2 -items , right?
Filter('SharePoint Folders', Approver = ComboBox3.Selected.Approver)
And in my understand , the 'SharePoint Folders' is your SharePoint list:

Your Combo box 1 -items use this code:
If(!IsBlank(TextInput1.Text), Distinct('Folders',Title)) This code return a single column table which columnName is [Value].
So now you want to filter the Combo box 1 items based on the Combo box 1 selected item.
Can you try to use this code in Combo box2 -items in your side:
Filter( Distinct(Filter('SharePoint Folders', Approver = ComboBox3.Selected.Approver).Title , Title) As items , !(items.Value in ComboBox1.SelectedItems) )
If this can not show data , can you check if this code can return data when it put on the Button-OnSelect(You can select all the code to see the data):
Distinct(Filter('SharePoint Folders', Approver = ComboBox3.Selected.Approver).Title

And what's the code return in your side:
ComboBox1.SelectedItems

If this reply can help you , you can click mark this reply as solution (Accept solution) which can help more people, thanks in advance!
Best Regards,
Yueyun Zhang