Trouble adding dropboxes in a textbox etc

Hello friends, I have a form with three dropbox and five textboxes

what I need to happen is, 

1. when document is loaded, only first dropbox should be enable others not, and after selecting value from first dropbox next should be enable and similar for third one.

2.on selecting value in first dropbox it should be on the textbox, "lbl2" and when i select next dropbox it should be added with first one and shown on textbox "lbl1" and same for the third dropbox (in the shape of percentage with symbol(%), formalu for percentage is ((cmb1+cmb2+cmb3) x 100) / 3

3. The values of textbox "txt1", "txt2","txt3" should be added on textbox "lbl1" in the shape of percentage with symbol(%) (formula of percentage is ((txt1+txt2+txt3) x 100)/ 3

Thanks in advance, for great help