lock after last signature

I will try to answer to the best of my knowledge,

• Can the document be altered by the first three signatories ? I can't seem to protect the document and then add signature boxes as I receive an error message.

There are a few onvenient ways to protect fillable portions (field objects) of the form. The best approach is to Certify the document with an invisible signature. You first create this signature using the "Certificate" tool. Once you've created the certying signature field you can set which portions you want to lock after you apply the certifying signature. To do that use the "Prepare Form" tool , rigt-click on the signature field and select "Properties" from the context menu. Go to the "Signed" tab;  you'll see a few options that you can choose from. Soon after you sign,   those field objects that you selected will be locked,  and you can now send the form. Also, you may need to protect (encrypt ) the document with password, to restrict the document from being edited, and additionaly, it would be a good idea use the  "Save As Other" option when you save this document to enforce Reader enabled rights. 

• How to I lock the document after my signature, the last signature ? Do I need to make my signature certified ?

To lock a document after yuo apply your last signature (not the certyfying signature-- remember this one will be hidden and protecting the form from users messing around with fillable portions that don't belong to them), you follow the same steps as described in my reply above. And no, if you follow this workflow applying another certifying (Visible) signature is not essentially necessary, but I personally think this is actually a good idea.

• If yes to 2 above, how can I make my signature certified but keep the other three as basic electronic signatures ?

What I would do in this case, I would not send this document via email as an attachment. I would send it out with Adobe Sign. 

This will allow you to strictly specify a signing block for each recipeient and lock it just for that recipient with an email ID authentication method (plus password to open document if you need that too). So the signing block for each user can be set like that and just for that particular use (nobody else can sign it that users block).

Adobe Sign will allow you to set the signing order too; Who signs first, who signs next , and who will sign last (in this case, yourself).

The nice thing about employing Adobe Sign in this workflow is that if you spot last minute errors after you've send it out,  you'll be able apply  corrections while the document is in transit.

This means, that if recipeint 1 already signed its portion,  but recipeient 2 hasn't signed yet, and recipient 3 is actually filling out its portion (on real-time),  you can still perform the necessary ammendments to it and it will update the document while it is still in transit.

I hope this answers your inquiry in the best way possible.

Just remember, that you can also set javascripts and resetting (or clear field objects) when you remove the last signature.

The will make the form editable again.  All parts,  or certain parts of the signed document, may be unlocked an editable again. 

Last, you can also create a template (or many templates) from the original document with Adobe Sign. And even publish this form online as a web form (if that is more convenient for your signers).

This way you can  re-use the same document many times with other signers or in modify it to the needs of other workflows.