First off, don't do it like that. Your code leave you open to a deliberate or accidental SQL Injection attack, which could easily destroy your whole database. Use Parametrized queries instead.
Secondly, please use sensible names for controls: You may have a good idea what data is in "textbox2" now, but I don't - and neither will you in a months time! Call it "tbAddress" or "tbProductDescription" and you code becomes much more readable.
Thirdly, your syntax for the SQL INSERT is wrong. You need to specify the columns in your table.
Putting these together, try this:
Com=New sqlcommand("INSERT INTO pro (Column1, Column2, Column3) VALUES(@C1, @C2, @C3)", con);
Com.Parameters.AddWithValue("@C1", textbox1.text);
Com.Parameters.AddWithValue("@C2", textbox2.text);
Com.Parameters.AddWithValue("@C3", textbox3.text);
Where "Column1", "Column2" etc. are the field names in your "pro" table. You may also with to change "@C1", "@C2" and "@C3" to use sensible names as well - I can't because I don't know what you fields contain...