How to filter data from a single table using 3 comboboxes as filters?

You are starting a single quote but not closing it

SELECT * FROM dv_principal where MOTIVE='" + cmbMotive.SelectedValue + "' AND CIV_ID='" + cmbCliente.SelectedValue + " AND DUE_DATE= '"+ dtpDueDate.SelectedText+"'"

Implement some basic debugging and look at the string you are creating

SELECT * FROM dv_principal where MOTIVE='Speeding' AND CIV_ID='15441151 AND DUE_DATE= '4th May 2018'

See the mistake? If a value is numeric you don't need the quotes at all

SELECT * FROM dv_principal where MOTIVE='" + cmbMotive.SelectedValue + "' AND CIV_ID=" + cmbCliente.SelectedValue + " AND DUE_DATE= '"+ dtpDueDate.SelectedText+"'"

You should look to use parameterised queries also as your code is open to SQL Injection attacks, and given the obviously sensitive nature of the data it's something you need to ensure is protected.

PS Note that if your response to this is "still doesn't work" that does not give anyone enough information to help you, your handling of dates looks suspect so that would be the next thing I'd look at.