How to fill different ComboBoxes in a DataGridView control with C#?

I am sorry I can't help much but I think you are describing cascading data sources. Look for articles in this web site about that.
 
It is relatively easy to do using bound controls but you need to learn about bound controls if you do not already know. You did not state explicitly if you are using bound controls. If you are not using bound controls then the solution seems easy to me; you just need to handle the event of the selection changing for each ComboBox; do you know how to do that?

Using bound controls, generally speaking, you need to handle the event of the selection changing from a ComboBox and in the handler function, set a filter (see BindingSource.Filter Property) for the data source for the next ComboBox. So when ComboBox1's selection changes, set a filtter for ComboBox2 and clear ComboBox3, or something like that.

Yes I read the link but this isn't the solution. Here are rows are removed and then added. But the rows are already existing and they can't be removed because the previous has already values.
 

This doesn't work -->
dataGridView1.Columns.Insert(0, ComboBoxDB);

am not sure.. try.. also refer links

       private void dataGridView1_SelectionChanged(object sender, EventArgs e)
        {
            if (dataGridView1.CurrentCell.ColumnIndex == 0)
            {
                
            }
        }

Just to clarify, my understanding is that you don't know how to make a certain selection of the combobox items get the proper data from your database (algorithm-wise, not syntax-wise). I'm really sorry for all the confusion if that's not true.
You understood it wrong. It is clear to me how to fill a combobox, What I don't know is how to fill them differently each row.
So as in my example: row 1 --> combobox1 has the value abc, def, hij --> combobox2 has the value table1, table2 etc... (when abc is selected in combobox1) combobox3 has the values: name, address etc... (when table1 is selected in combobox2)
row2 --> combobox1 has the value abc, def, hij --> combobox2 has the value TV, Radio, Laptop etc... (when def is selected in combobox1) combobox3 has the values: button, speaker, etc.. (when Radio is selected in combobox2).
So putting values/items into a combobox is not the problem, but putting different comboboxes in each row is.

I don't know much about your project, but from what you've said I assume you have a lot of data, so if you don't want to torture yourself (bad code is bad for your mental state - think about maintenance), you should make things organized, and I think MVC pattern suits well here.
I'm wet behind ears myself, but I'm sure someone much more experienced will tell you too that it's good to separate GUI, data and the connection between GUI and data.

Just to clarify, my understanding is that you don't know how to make a certain selection of the combobox items get the proper data from your database (algorithm-wise, not syntax-wise). I'm really sorry for all the confusion if that's not true.

It might help if you upload your project and a sample of your database.

About putting items into combobox, use Items property, which is a collection-type:

my_combobox.Items.Add(...);
my_comboboc.Items[0] = ...
..etc..

You need a good design first, something that will dynamically populate your comboboxes and your datagridview, and I think delegates can be useful here. Think of delegates as function variables. You can use them anywhere (not that you should).
I still don't see why to use delegates and why a good design?
It's simple to put a DataTable in a Combobox and the design is clear because there are a few databases with tables and this tables has columns, nothing more nothing less. Also with Delegates, please explain me how to put values in each Combobox because that's where the difficult is. If I know how then I can also populate them with the values from my DataTables.

You have a LOT of combinations to worry about, over 1000 (10+ items per combobox). You need a good design first, something that will dynamically populate your comboboxes and your datagridview, and I think delegates can be useful here. Think of delegates as function variables. You can use them anywhere (not that you should).

In my opinion you should really focus on the algorithm first, leave the practical stuff, such as filling your datagridview, for later. I myself would implement model-view-controller pattern.

Then its very simple to implement in Javascript.
You can achieve this using JSON call.
Based on the first combo box selection, JSON call will hit the database and file the second combo box.
 

May I Know how many items will be there in the combo box approximately ?
I can't say, It depense on how many tables a database has. Because it show's the name of the tables and after selected on of the tables it shows the columns. I think that companies don't have databases with more than 200 tables so say the values will between 1 and 200.

May I Know how many items will be there in the combo box approximately ?
Becuase if the items come around 100 to 200, we can have it in array. Based on the selection, we can select or fill the items in other combo box using the normal javascript.

I can't use delegates because the values are comming from a DataTable from a database. This also means that some Comboboxes has more then 3 values, normally did will be more then 10 values for every Combobox. The above was an example to make the question clear without to much overhead.
I tried code like this -->
dataGridView1.Columns.Insert(0, ComboBoxDB);
dataGridView1.Rows[rowNr1].Cells[colNr + 1].Value = Convert.ToString(cmbBox.SelectedIndex); 

What ever I tried It doesn't work, so please send me code how to do it.
I only think that it's not possible what I want? But if you think it's possible please let me know.

Have you tried using ComboBox's SelectedIndexChanged event? I've done something very similar before, where I displayed a different set of controls depending on which option the user chose.

But be careful when writing the whole thing... For example, 3 options in each ComboBox will give you 27 different combinations.
This can quickly turn into a spaghetti code, so I recommend you to use custom delegates or events, and separate code heavily into functions to avoid any copy-pasting and to make it possible to dynamically construct one large function out of little ones.

This article might help you with evaluating multiple variables, but I don't recommend making a switch with 27+ cases.

Upload your project if you get stuck.